And yes, you are right, 4:15 seems to primarily indicate prostitution, though this is not the only form of major fahisha.
See the end notes for Brother Joseph's article on Homosexuality.
The phrase 'until God ordains for them another way', in 4:15, in my opinion, constitutes an acceptance of possible repentance on part of the women. (Also see end of Joseph's article on abrogation)
Also, just because the punishment in 4:16 is undefined, this does not signify laxity. Undefined means just that--undefined.
Regards
wanderer