As someone who is studying psychology, philosophy and politics and previously wanted to do neuroscience, you have three options:
The first is to do a foundation year as others have suggested. Believe it or not, there are a lot of people who don't study STEM A levels but go on to do STEM degrees. Personally, I think this could be a good option if you are heavily invested in your current choices. Doing a foundation year does mean an extra year at university, but if you think you will do well in your current A levels and you are keen to learn more about them rather than replacing one for biology, it might be a better route. If you are between studying neuroscience and something else, this may actually help you make your mind up. I didn't get the grades needed for biology so I switched that to English last minute and hated it. I then switched to politics because I had a bigger interest in politics than biology and I was also considering law as a degree. Studying politics and philosophy made me realise that I'm not as invested in psychology and neuroscience as I thought I was, which helped me to decide on studying law. I also really enjoy how politics exams are done, as they are very similar to history exams with (in my opinion) more interesting topics.
Your second option is to start off with four A levels and drop the one that you don't want to do after the first few weeks or months. Some schools won't let you do four A levels if you don't have very high grades, but this varies. If possible, I would try and start with four (the four being politics, philosophy, psychology and biology), as this will help you make up your mind for certain. It also gives you a little time to figure out whether neuroscience is really for you. This is probably the best option.
Your final option is to replace one of your subjects for biology. It can't be psychology as for neuroscience, many universities will require biology or chemistry and an additional science-related subject, which psychology is classed as. It would have to be either politics or philosophy. I really love both of these subjects, but if I was in your position and my heart was set on neuroscience, politics would be the one I would switch. Politics is actually my favourite subject and I perform very well in it, but out of the two, it has a lot more content. Biology and psychology are already content heavy, so to add politics on top of that may make your workload a lot heavier. With philosophy, assuming you're doing AQA philosophy, there isn't actually a lot of content, however it is hard to understand the content there is sometimes. I'm lucky that I have great teachers, but I struggled a lot in my philosophy exams throughout Year 12. It was only in my end of year mock that I achieved a high grade, and even then it was still a struggle at times. You have to really understand the content to do well. Personally, I wouldn't go down this route unless you are
very sure you want to do neuroscience.
It's not an easy choice, but you did pick these A levels for a good reason (I'm assuming). As others have said, neuroscience won't necessarily offer you the best job prospects if you decide to stay in that field, but your degree won't necessarily dictate your career. You could go into pharmaceutical development or work in drug testing. You could do a conversion degree and go into psychology to eventually qualify as a neuropsychologist. You could take your neuroscience degree and go into a completely different sector, such as bioinformatics or marketing. Every degree has value if you make use of it while studying. I would suggest looking into the modules in different neuroscience degrees. This is what helped me realise that neuroscience might not really be for me, as I was more interested in human behaviour than actually studying the brain. From there I began researching more into psychology and as much as I think I could enjoy it far far down the line, politics had a big impact on me and I knew that studying and working in psychology for the next decade was not the life I wanted. I'm still interested in human behaviour, but more so how society shapes human behaviour, which is what drew me to law. You should also be aware that in a neuroscience degree there is a lot of chemistry, and as someone who hated chemistry, this put me off heavily.
If you have any more questions I'd be happy to answer them