Trouble with getting such splitters as RJ14s (4 pins) instead of RJ11s (2
> pins) is they're damn-near impossible to find. In the form-factor I need,
> anyway. And I don't want to buy one unless someone can explain to me why
> the 4 pins are needed. Any help appreciated.
If you're plugging a modem into it, you don't need the outside pins. Most phones that will plug into it don't need (or use) those pins either.
In the USA, those pins would be for a second phone line using the same
4-wire building wiring. There are 2-line telephones that make use of all 4 wires. There are splitters that take one 4-wire plug and turn it into two independent 2-wire receptacles (each a different line). I don't know anything about UK telephones, but I'd guess that if you only have one line, all of that is irrelevant to your usage.
Dave