I don't recommend sharing a 56k connection between four computers. If two or more of the machines tried to access the connection at once the shared bandwidth would mean that everything would slow to an unacceptable crawl - the telephone line simply wouldn't be able to deliver analogue data fast enough.
If you still want to go ahead you'll need to buy a router - some of the ADSL models have an RS-232 port into which you can plug a 56k analogue modem.
Then you simply need to add a wireless network adapter to each computer. The simplest way of doing this is to use USB adapters - there are many different models at reasonable cost. For the laptop you can get a PCMCIA type adapter, or use the same USB model that you use for the PCs.
It all works very well, and the hardware is easy to set up.
I do urge you to think carefully before committing to a couple of hundred pounds worth of expenditure though, because you may be disappointed with the result.