To be honest with ya there is no improvement between these models thats worth the price difference for the tried and true 4100 and its replacement (the 2320). At best its a few hp at the PTO but no increase in hydrolics....the same thing with the 4115 replacement 2520, even less of a HP difference, and no increase in lift ability.
I wouldnt even consider the 2305 since you have a 4100 which is the superior machine.
The 410 loader was the bench mark for the 4010 thru 4115 series. I doubt there are many around but the 200cx is curved and has a little less lift ability than the 410. The curved arms come in handy when the tractor hood is curved like the 2320 & 2520 series. Not sure if you get that benefit on a 4100 with no curved hood.
If anything I would be shopping for a used or floor model of the 4115, it would be less on the wallet and get you where you wanted to be without giving up anything...