Check this out:
Emu 1212M PCI 24-Bit/192kHz Balanced Interface (Windows) from zZounds.com! a card from E-MU, one of the cheaper ones out there and it has 4 inputs I believe
M-Audio also makes 4 (or more) input audio cards but because they are made for high quality sound recording, they tend to cost way more than something like the above.
In the past, I have used the Lexicon Omega which comes with Cubase LE (a multi-track recording software).
Lexicon Omega from zZounds.com! This puppy is external and works via USB. It also has 4 inputs that you can route to individual tracks in whatever software you are using (like the cards mentioned above are capable of).
If you are not trying to do multi-track recording, you can find a cheap mixer (such as something Radio Shack may sell) and run a single input into your existing sound card. Unless you are recording a bunch of transmissions at once (why?) then the mixer lets you choose what channels to record and at what levels right on the board.
In the end, I would just go with a cheap/decent multi-input sound card (internal or external) and get a multi-track recording package (if it does not already come with the hardware). There are lots of programs out there should you need one so I can not honestly begin to suggest one as I use M-Audio Pro Tools for music recording.
Good luck on your search and project.