Greyfields - I assume to insert "5105" wherever you say "4105". Two comments, as the owner of a 5105 who shopped the 4x20 series also:
-the weight difference is probably more like 1000lbs. I think JD rates the 4120-4520 at 3750lbs. JD doesn't publish a weight for the MFWD version of the 5105, but it appears to be around 4700lbs. The 522 FEL on the 5105 will weigh a bit more than the 400x loader on a 4520. I figure the weight of my 5105 with FEL and filled rear tires to be about 7000lbs. This limits the ability to trailer it - without a 3/4 ton or larger pickup, trailering is almost out of the question.
-JD used to publish a questionable lift rating of 4800lbs for the 5105. The NE tractor test report shows something more like 3000lbs. I think 3000lbs, or the 2500lbs on a 4520, is more than enough for any implement you would appropriately use with either tractor. Of course my 5105 never strains to lift any implements I have, the heaviest of which is a 6.5' disk weighing about 900lbs.
Fuel consumption will be lower on the 5105. I am burning less than 1.0 gal/hour in the 92 hours of use I have, and I am using it fairly hard for tillage and mowing.
I would add that while they have the same tire options, realistically most 4x20 models will come with R4's and most 5105 models will come with R1's, in either case suited to the intended use by most owners.
None of this automatically extends to a 5203, since it is a somewhat different model. In particular, the 03 series has turbo engines and are made in India. For reasons unknown, the 5105 makes more power than a 5103, even though the 03 has a turbo and the 05 doesn't.
My wife and I both learned to drive manual-shift so that wasn't a big issue for us. If you don't have that experience, it might be a bigger issue.