256kbit down i assume u mean, which means 32K max download speed (kbits ar 1/8th of Kbytes aka 256 / 8 = 32)
This would equate to your 28K known download speed (giving a bit for transmission maybe some over head here or ther etc)

Now the reason your getting at 4Kbytes a second, i have a feeling might be that the data is comming back in the dialup connection, i havent seen how saterlite creates a dual return pipe, does the saterlite need to be accessed by way of proxey services or is it a transparent device that needs no settings for it what so ever?

I would not consider 4Kb acceptable even on dialup as it should be 5Kb+ only 1 difference but 1 here is 25% more!

What is the result of you starting a second get, does each get go at 4Kb or close to it, or do each half to around 2Kb meaning your totalling only 4Kb?

If it is the first (4Kb each) then it may well be the outbound speed you have to acknoledge packets (do you see a high speed for a few seconds at the start of the download?)

if its the second (2Kb each) I would think its not using the saterlite to bring the info back in, rather its comming in the modem (this might be a limit of what ports the saterlite returns info on, with other ports being returned through the modem, if so then u could set the dcc ports in mirc to use a range inclosed in that range of the saterlite)

Sorry i cant give any closer advise.

PS: always a chance im completely off track as well sorry. Hmmmm Sun spot activity? LOL!