I have the 8 3/8" barrelled 500 with two different compensators, and excellent X-500 Hogue grips. My top home-brew load is 41.5 grains of 4227 with a 350 grain Berry's plated bullet dropped on top and all sparked with a large rifle magnum primer. Recoil is 'brisk'. And I like the daylight visible fire ball. This revolver is 75 ounces. (The same gun with Hornady factory loaded 500 grain XTP's is...ugly.)
I think that a 4.2" barrelled 500 in comparison to the 8 3/8" would be like the difference between my S&W 69 (37.4 oz) and S&W 29 (47.7) when firing maximum loads with 300 grain XTP's... controllable, but with more barrel whip / upward recoil. The 4" 500 is 55.6 oz vs 75 oz for the 8 3/8".
A 4.2" 500 would be quite controllable with 350 grain jacketed bullets up to 400 grain gas checked hard cast. I would probably avoid the 700 grain 'Tyrannosaur' loads, however and those nasty Hornady factory 500 XTP rounds
I have lobbied S&W a number of times for a 4.2" 500...I just need 98 other committed buyer's out there!