Been looking at them as well.
I think the sb4200 has a few more whistles than the sb4100,but they are both docsis 1 and docsis1.1.
Not sure exactly what that means,but i do know that docsis 1.1 is only just being rolled out.
Dont think either will be out of date anytime soon as they would have to replace them all.
I was always put off them because they look so 1970's.
I see them on ebay and the plastic has turned yellow!!!
The sb5100i's/e's are capable of docsis 2,but who knows when that will be.
I think that's when you can have a 30mb or something like that and i've read that it would be difficult to do the way the system is because of the distance of the main telephone exchange thing???
Get what evers the cheapest and then look out for a sb5100i.
They look more 21'st century.