they are almost exactly the same amp. the v4 has a lo switch (which almost no one ever uses) the mid eq points are the same, and the v4 has input sensitivity switches, the v4b doesn't. for more info, take alook at this page: http://stonemarmot.com/rants/?p=38
The link that John K sent you answers that question, but,basically, this is the head separated from the speaker. In other words, The vt22 is the combo version of the V4 (amp only).
V4's say VT22 and V4 on the back of them (because they're the same amp in different cabinets).
here's the back of my VT-22 (look above 'the ampeg company'):