I'm thinking of buying only UT2004 since it's only $4. Around how many players are still active on the game daily?

I might be overly optimistic. But is it's active playerbase anywhere close to Halo's on pc? (Several hundred, may get near 1k at points.)

I just finished installing the game and launched it. Here's a screenshot of the server browser as of right now with empty servers hidden. It's worth noting that it's pretty late (California), so this probably isn't representative of peak hours, but it seems that while this game is (expectedly) not as popular as it was when it was launched, it seems populated enough that you shouldn't have any trouble finding a game to play.

It's definitely worth the price tag.