I have the same question as I too own a 4th gen (and an IM3) and am considering a 5th gen (30gig).
The following link says that the IM3 was made for the 3rd & 4th gen ipods.
Over time, not having an ipod sitting properly in the IM3 can only lead to unnecessary wear on the docking connector of the ipod.
I'm planning to contact Altec Lansing to see if they will offer adapters and will post the info when I get it. The IM3 is one of the better mid-range prices speakers available that also has battery power as an option.
http://www.alteclansing.com/downloads/USIM3_SELL_SHEETPA.pdf