well with a 4 gallon you don't have to change the water that often. You could go about 2 weeks, do a 50% change, then go another week and a half and do a 100% change.
I'm not a big fan of UGFs...they stressed my fish out....personally I'd take it out, but that's just me..it's really that's your preference how to filter it. With a filter in there, and a UGF at that, your water change schedule would actually change..but I'm afraid, not knowing alot about UGFs, that I can't offer advice on that.
As far as the sun goes..if you wanted to try it...they have those grow lights at walmart...you could toy around with one of those if you really wanted too...but I don't think that is your problem here. I just read that it was beneficial to them is all I meant by that part of my post...I'm not even sure "how" acurate it is.