IMHO you don't need more then 10x for Big Game hunting. I have no problems using 9 or 10x scopes and shooting to 500 yards. I also see little need for a 50mm objective, I use to think otherwise butI foundthe possible negatives out weigh the benefits for most when it comes to big game hunting hours.

Some of the negatives:
The scope must be mounted higher which can cause for poor cheek weld(a component that can effect accuracy)
Higher can mean it won't fit into carrying or storage devices
Higher easier to get snagged on brush/limbs
Highermounts can lead tomore fatigue on the mounting system
50mm objectives cost more socan lead to one choosing a lesser model optically to stay within budget and obtaining less performance

As far as the FOV goes yes it is wider in a comparable model 50mm vs 40mm, however in all honesty it has never amounted to much gain in the field IME. Though it is a personal preference so you must deciede this for yourself. I have heard guys say they feel like a fisheye looking through a 40 vs larger???

In regards to the scopes you mentioned I'd go with 4200 in 40mm or a VXII in a 40mm. I have a VXIII and honestly wouldn't buy another at this time. In fact I suggest you look at a Zeiss Conquest as well. A 40 mm will run $400 or less. You could also check out a 44m, though htey are more money. I have both of these Conquest models and they are superior to the VXIII, VXII and 4200's I have or looked through. I would save the money personally and buy a 3x9x40 Conquest but again that is for you deciede not me