One of the biggest problems with 4th species counterpoint is that you will do chains of random suspensions, divorced from harmonic context. Though the skill of working under strident conditions and coming up with the best solution possible is important for fugue, you also need to marry that with making good harmony, otherwise you will not be ready for fugue. Watch this full short tutorial from Preparation for Fugue, Bass Exercise #1, with 4 suspensions. Best of luck with your fugues. -Barney