Adding UCAS courses after applying

If you submit your UCAS application with 4 choices, can you add your 5th later?

I'm thinking of applying early with 4, then adding my 5th choice more strategically based on which offers I get(Or lack of) towards December/January time.

I've read this is the case, but can't find anything 100% definitive?

Yes that is the case. My daughter did it this year so that she had some interview practice from one choice before adding her other choices in December and January. A long as you add your other courses before the January equal consideration deadline, it is fine.

If you submit your UCAS application with 4 choices, can you add your 5th later?

Yes.
Just make sure that you add any other choices before the January deadline but be aware that you may not have any/all decisions on your original choices before that point.