Go for it! I brewed an ag partial boil Bpa with a 4gal boil and everything went well. I mashed with a 1.25 water/grain ratio and batch sparged to hit 4gal preboil. Plan for low efficiency due to the lack of an adequate amount of sparge water, mine was 60%.
I just re-read your post, i wouldn't top of with sparge water. Either do a split boil or do it the way that i did and plan for low efficiency.