Both the OU and Birkbeck's degree should be based around approximately 3600 hours of study because this is the standard which all UK universities need to adhere to when designing the structure of their courses. So yes I would imagine a 4-year degree would undoubtedly be more intense than a 6-year degree.
Most on-campus courses tend to run for around 30 weeks of the year (the other 22 weeks are Summer, Easter and Christmas breaks). Each semester tends to be 600 hours spread across 15 weeks = 40 hours per week for a full-time student studying a 3-year degree.
If you're going to spread 3600 hours across 4 or 6 years part time, then you wouldn't be including such long study breaks, so that'd be either 200 weeks or 300 weeks, then you're looking at around 18 hours per week for a 4-year part-time course, or 12 hours per week for a 6-year part-time course. (These are rough numbers of course).
There are other options you could explore too - firstly, if distance learning interests you then I'd recommend looking at other non-university courses; particularly those which lead to industry-recognised certification. For example, there are quite a few paths available from Microsoft, Oracle, Cisco and Amazon which you can study in your own time to gain competencies in specific technologies.
Secondly, there are online-based courses, many of which are completely free, or available quite cheaply, from websites like EdX, Coursera and Udacity. These sites offer degree-level modules from global top universities (e.g. Harvard and MIT) as well as top tech companies like Microsoft, Amazon and Google -- most of which you don't even need to pay for since you can ignore the MOOC certificates. The advantage of taking these would be the ability to schedule your own learning based entirely on your own schedule, and focus on the skills which are specifically relevant to the types of jobs you'd like to apply for.
Obviously these won't lead to a traditional degree qualification, althoug hthe value in going to university is not in obtaining a degree, but in spending 3 years learning a solid set of the skills which employers want to see in the people they're hiring into junior/entry-level jobs -- realistically anything you'd learn on a degree can be self-taught using MOOCs without really needing to pay much or anything.
Most employers have absolutely no preference for people with degree qualifications (Unless the employer is specifically looking for academics to fill a research-based role), although they'd want to hire people whose skils are up the same minimum standard as a graduate. Recruitment and interviewing procedures are nearly always focused on assessing individual candidates based upon technical, analytical and problem solving skills, as well as evidence of having used real technologies in their own projects, or having worked on projects with/for other people, and any other relevant experience. So - focus on the knowing which skills you need, and learning those skills, rather than worrying about a qualification.
Another possible option could be to enrol on a higher (Level 4-6) apprenticeship scheme (DIgital & Technology Solutions) - this focuses mainly on picking up skills while starting out with that kind of job from day one. Usually 4 days a week working for an employer in a discipline such as Software engineering or Platform engineering, alongside one day per-week studying towards a degree qualification. The advantage of doing this is that you'd learn a lot more this way about the specific career path than studying on a degree; by the time apprentices 'graduate' from the scheme, they're at a mid-level with 3 or 4 years of experience to support them.
With that said, the apprenticeships schemes are competitive to get into, so you boost your chances by self-teaching via Coursera/EdX/Udacity and other MOOC websites then treat the apprenticeship scheme as a target/goal, although the people you'd be competing against will usually be straight out of A-Levels or Level 3 BTEC courses, so the bar is lower than university graduate jobs.