4 phones or 4 lines? There are a lot of cordless base stations that can support more than 4 phones, and even multiple lines. The Siemens phones recently being discussed in the VOIP thread for instance. Plus phones from Uniden. I think their bases support up to 10 phones, and they offer models with at least up to 2 line support. They should work up to 250 without much issue, especially the lower frequency variants like the DECT models.

Other than that, I suppose you might be able to find something to let the analog signal go over your data cabling. Don't you also already have some copper run with your fiber?