5th option?
I can understand you taking 4 A levels with a view to drop one if it gets too much, as that’s quite normal, but considering a 5th subject would be rather mad.
Honestly, the three sciences alone should be a decent combo for dentistry. I generally advise against taking 4 A levels unless maths and FM happen to be two of the four A levels taken as the overall workload is generally more manageable that way. If you were to take 4 subjects for dentistry, I would recommend swapping out physics for maths and taking FM as the fourth.