I don't see much difference between the 4300 and 4400 tractors except HP, especially since you are getting HST transmission (the standard tranny in a 4400 is a 12/12 synchreverser, which is an option on the 4300). Whether the 3 HP difference is significant may depend on what you hook to the PTO, I can't imagine it being a real issue for pulling power (traction is usually the problem on compacts).
I'd take a look at the requirements of the
Chipper you are considering. While 3 HP isn't necessarily a make-or-break deal on a
chipper, if the one you want has a HP range that pushes the limits of PTO hp on a 4300 (25.5 PTO HP w/HST) then I'd be inclined to bump up to the 4400 (28.5 PTO HP w/HST). I didn't see much else in your plans that would push the limits of your HP. Either will handle a 5 ft brush cutter without much trouble.
420 Vs 430 Loader: (can be mounted on either 4300 or 4400)
(1) 430 has a higher max lift height (101.4" vs 87.6") This might be a factor if you were trying to dump into the side of a truck, for example.
(2) 430 has a higher rollback angle on the bucket (30? vs 24?) not sure if that is significant for you.
(3) 430 has a significantly higher breakout force (3299# vs 2693#) and a slightly higher lift capacity (1258# vs 1195#)
(4) 430 is slightly slower than the 420, but not noticably so (0.1 or 0.2 seconds on most specs). The only significant difference is in loader raising time, which is to be expected, since you are raising it higher (about half a second longer on the 430)