Will stopping PRP after 4 sessions lead to more hair loss? (photo)

First, I would recommend a full work up and blood work to see if there is a cause for your hair loss. PRP can help decrease shedding and increase hair growth however, after the initial 3-4 monthly treatments maintenance PRP treatment is recommended every 6 months.

Hair shedding is a normal part of the hair growth cycle. It occurs because hair only will grow for so long before it "recycles". The mature hair is pushed out by a new hair and growth goes on without thinning. Excessive hair shedding can lead to thinner appearance and stimulating the follicles to produce hair can help. That is where PRP comes in. If you are treated with PRP for several months you will see increased hair density. If you stop your follicles will return to their normal state, as it was before you started, and thinning will reappear. You are not worse off for having done the treatment; you are just back to your baseline.
Of course there are medical treatments for hair thinning like minoxidil and finasteride as well as supplement like Nutrafol. These can be very helpful and I recommend a combination of treatments be used for best effect.