Although three a-levels is usually all that's necessary, four would seem more impressive, but this depends on whether you'd continue to do it into A-level or just stop at AS. In this case it may be better to pursue French outside of school and do an EPQ in a subject of your interest in year 12. This means you could link the EPQ to a subject you might be thinking of studying at uni, while at the same time improving your french. Perhaps at the end of year 13, you could enter some French-based course that could give some proof of your ability to speak/write French?
If you don't need A-level French for your future uni plans, then I'd suggest not pursuing it in this case. Although technically if you wanted to maximise your options for potential degree choices, then it would be better to take French over psychology since the latter is not a requirement for any degree programme. However as you noted, language A-levels are (apparently, I didn't do one) a significant jump up from GCSE and require a lot of ongoing work to keep on top of things.