To answer (as best I can; I'm not a WotC employee, just going off what's been publically released) the OP's question, which seems to me to be a perfectly reasonable one...
The power curve in 4E is a lot flatter than it was in 3.X; roughly speaking, one 4E level is approximately one-half a 3.X level. So the game should tolerate level gaps better than 3.X did. A gap of 1-2 levels should be no problem at all. 3-4 might be pushing it.
On the other hand, since the system is back to fixed XP awards, there's no longer the "rubber band effect" of 3.X; lower-level characters won't be "pulled forward" and catch up. If one PC consistently gets smaller awards than the others for any reason, that character will fall further and further behind. I don't know if 4E will have a system for dealing with this; if not, you may want to house-rule one.
The power curve in 4E is a lot flatter than it was in 3.X; roughly speaking, one 4E level is approximately one-half a 3.X level. So the game should tolerate level gaps better than 3.X did. A gap of 1-2 levels should be no problem at all. 3-4 might be pushing it.
On the other hand, since the system is back to fixed XP awards, there's no longer the "rubber band effect" of 3.X; lower-level characters won't be "pulled forward" and catch up. If one PC consistently gets smaller awards than the others for any reason, that character will fall further and further behind. I don't know if 4E will have a system for dealing with this; if not, you may want to house-rule one.
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