While i was doing a bit of research for the surround speakers, i came across this article on the techtree about the VS 4121...the only difference between the Atp3 and the VS4121 is tht the VS4121 sub is rated at 19watts as against 18watts of the atp3 sub...apart from tht the vs4121 has a headphone jack which was missing in the atp3...Also, according to the review, the vs4121 is slightly louder at peak volume than the atp3...
As for the VS4221, it has a remote and the satellites are rated at 8 watts against the 6 watts satellites in VS4121..so it shud give a slightly better high n mid range performance than the VS4121...the sub is still 19 watts like the VS 4121....
btw, if u know the price of the VS4221, do let me know..