Trying a 4 star map won't affect how much PP you gain from lower star songs.
What does affect how much you gain is your rank. Once you rank up enough lower star songs will stop giving you ranks. BUT, that doesn't mean that you have to move on, especially if you aren't ready for the higher level songs. You can still practice on lower 3* and you will gain skills that translate into 3* and 4*.
Another reason you might not be getting rank is because you aren't getting high enough accuracy or combo on a map. If you break combo at any point or your accuracy is not high enough it won't give you enough PP to affect your rank.
Hope this helps.