Applying for Master's Degrees 4 years after completing undergraduate degree? Advice

@threeportdrift - have some good and bad news and was wondering if I could seek your advice. Good news is that one former lecturer has agreed to provide me with a reference, however the bad news is that I can't seem to reach another one. There's maybe one more who taught me that is around this semester (there are a few more who taught me but are either on sabbatical leave or maternity leave so not currently around) but I'm not sure how much longer I should wait until reaching out to them.
I sent a follow-up to the second lecturer I'm still waiting for (and they would be my preferred since I did do very well in the modules they taught me) but I'm not all that hopeful for a response. How long would you recommend waiting before sending a further follow-up or just moving onto ask another lecturer? My worry is that it'll be awkward if they respond very late if I get agreement from someone else, and also that both may not respond to me at all.

Just work back from the date you want to submit your application. "I've been advised I should submit my complete application, with references, before the end of November", then go back to the one you are waiting for and ask them if they can meet that timeline, or would they prefer you asked the other person. Then if they don't answer that, you can go to the other person, and they can't feel sore about it.

Two weeks minimum for a reference request, it's simply not a priority for a lecturer and will take some time to schedule and think about.
If someone declines, then you have to move on. You don't watt to get into an awkward situation about whether they just don't remember you, or they don't think you are worthy of a reference!

@threeportdrift in a bit of a difficult situation at the moment. So the lecturer who agreed can no longer provide the reference anymore because they have had to take some leave for personal reasons and it's not completely clear when they will be back at work - totally out of their control and they were very apologetic in the email back to me.
Now, I got a response from another lecturer earlier on in the week who straight up declined saying they don't know me so can't do anything about it. Fair enough I suppose, so I shall move on.
There's still a fair bit of time for most courses (maybe Jan/Feb is when I'd like to complete the applications for at the latest), but there's a couple in particular where I have just under a month left to apply - shall I give up on this one whilst I'm still trying to reach out to other lecturers?

Yes, reach out to anyone/everyone. Same deal as before, bit of a pen-picture to help them remember you, and help they say something positive, ie what you did with them, your best bot of work, explain the relevance of the new course etc. Mention the deadline. Email the admin at the same time.

Also, @threeportdrift, does it look odd if you have a reference from someone who didn't teach you? We had a great senior tutor (who actually ended up writing most references for people applying for post-grad study) when I was at the university but he left during my last year and I have no idea if the new one (who I don't know) could provide me with one.

Yes, reach out to anyone/everyone. Same deal as before, bit of a pen-picture to help them remember you, and help they say something positive, ie what you did with them, your best bot of work, explain the relevance of the new course etc. Mention the deadline. Email the admin at the same time.

Think I may need to give up on this dream now, had a couple more people say no and I'm running out of options ☹️
Bizarrely though, I did get asked from one of them if I could help at a careers session for undergrad students after they found out the details of my job from LinkedIn (kept it fairly vague in the email but the company's well known) 🙄 Can't believe the nerve of some people

Email the Head of the Department and explain the challenge. Be balanced, explain who you have asked, accept the time gap, but ask if they can resolve the issue otherwise you may not be able to advance your career.
Also, go back to the uni you are applying to, and ask about acceptable alternatives.

Think I might need some further help of yours - might need to PM you as I'll feel comfortable sharing further details of my situation rather than publicly on this thread, but will try and see how it goes here first.
I'm confused with all of these positions - is the Head of Department the same as the Head of the School (of my subject)? What's the difference between that and the senior tutor? When I was a student, pretty much no one had contact with the former, but everyone knew the latter. I'm guessing the Head of Department has more power and influence?
When writing the email to them, how much do you advise mentioning? Shall I go into more details about my work and emphasise that I am still working in the same sector and it's purely because my employer can't provide references which is why I'm asking? Think I may have to do this through admin too because I wonder if there's a filter with their email system that automatically sends all generic outlook/gmail/yahoo/etc. to their spam
I shall try and see about asking about acceptable alternatives to the universities I'm applying to - though I was also considering applying to Oxford and Cambridge based on recommendations from colleagues and my manager at work, but that has the earliest deadline.

You need to get tougher with all parties here. Work and your previous university are withholding information and support for your next degree. Look at the department hierarchy on the uni website, write to the most powerful looking person there - every uni has different job titles. Explain that you are trying to further your career, and persons X, Y and Z have not given references. Ask for any sort of reference.
Speak to a colleague at work to see if you can get a reference outside the system.
Speak to your applying universities about options if you are 4 years on from your degree, when so many people who knew you have moved on.
I got a reference from an undergrad uni 15 years later, and got into Cambridge, then 10 years later had to have a coffee with a Cam academic to get another reference. He simply didn't remember me by email, but as soon as he walked into the cafe he twigged and we caught up, and a reference was forthcoming.
A reference is a bit of a bellyaching job for academics, but on the other hand, if folks are successful, some of the glow reflects back on the academic and uni/course. So they should be supportive unless you were a complete pain.
You need to press people harder. You don't need to be rude, or upset people, but you've taken all the normal routes and they are just sloping shoulders.
Feel free to pm if needed