Just got one of the Rossi youth, matched pair models for our 9 year old daughter. She got it as sort of an early, combined, Christmas and 10th birthday present ((((birthday not until the end of Dec)))They have several different combinations, and adult sized models as well as youth sized models. The one we got her has a .22 LR barrel and a 410 shotgun barrel. I didn't realize it until I got the gun and noticed it, but the gun says both Rossi, and Taurus on it, both well known name brands.
Based on appearance and shooting it it's a well made gun. With the interchangable barrels it makes it very versitile. The good thing is that changing them out is as simple as unscrewing the sling swivel in the foregrip and breaking the gun open just like any other break action single shot. This is something so simple that a young shooter will have no problem doing themselves once familiar with the gun.
Out of the box her .22 still needs a bit of tweeking on the sights to get them dead on, but the 410 holds a pretty decent pattern out to at least 30 yards which is as far as I've tried it. The shotgun still has a bit too much kick for her to shoot, but she loves the .22. Both barrels offer auto ejection of the spent cartridge which is nice. They also have both the regular, in use safety as well as a special parental safety that locks the hammer with a sepcial key that comes with the gun.
Price wise I found them all over the board, but Wal Mart beat everyone else I tried on price by nearly $100 selling it for $169, plus tax.
The 4 year old will, more than likely, be a bit small for this gun, but there are other .22's on the market that are smaller in size that would probably work. That said, if it were me, at that age I'd be more inclined to get him one of the very real looking airsoft guns to begin teaching him basic gun safety before putting a real one in his hands.
Good luck and I hope your sons both enjoy their new gun, whatever you decide to get them, as much as our daughter enjoys hers.