Splitting a zip file into parts?

OK, I see a number or suggestions to split a zip file into small parts in order to allow a dial-up user to work within the 20 minute limit hard-coded into Zeb Cart.

Never having done that, I have no idea of how to do this, does the user download one at a time, and how do they reconstruct the zip file when they have all the parts?

And how does this impact the users who have high speed connections who don't need the splitting?

I am using jzip to do the zipping, the files are around 4 MB in size, and the user tells me he gets about 800 KB in the 20 minute download window.

I think WinZip can do that. I have seen sites where the customer was offered the choice between one big file or 4 or 5, depending on the size of the original.