256Kbps will be divided among the computers

It is true that your 256Kbps will be divided among the computers, but the key is each computer will only pull as much as it needs.
It doesn't mean that each of the computers will get 51Kbps. You might have one computer downloading an iso (cd image, about 700Mb) at a 100Kbps, and then the other 4 computers just doing pure html webbrowsing, and no one will know the difference. Especially since all 4 computers won't be pulling browsing at the exact same time. One person will click, and start reading, and then another, etc, they're not going to be sync'd.
Also, if you share or not, it will be transparent to the ISP, they can't tell, since the sharing scheme you're using is likely a NAT (network address translation). From your side, your computer sends all requests to the NAT box, and the box sends back to you whatever you requested. From the ISP's point of view, they only see the NAT box making requests and all incoming stuff goes to the NAT. The NAT box keeps track of what computer requested what, and returns the data to that computer. Naturally, it can get very complex, but thats the short version.
Either way, sharing seems like a fine idea, depending on what your users are doing. I know that I'm trying to get a similar set up as the one you're talking about. I would have about 7 computers on the network (but probably only half in use at a time), and only one person would be pulling down huge files (me), so I'm aiming for the same.
Hope it helped.