I've played the demo for both and I like both of them but I only have money for one. I see that Swat 3 is easier to get and I'm leaning towards it, but if there are any Swat 4 fans that say otherwise please do so.

This is a difficult question.

Swat 4 has a newer prettier game engine but in terms of Swat tactics Swat 3 is miles ahead of Swat 4.

Swat 4 has stupid magic doors that always open away from you. Simple things like this are the reason why Swat 3 is superior in terms forcing you to play tactically. If you are a person that cares about graphics more than gameplay then get Swat 4.

Swat 3 has a little bit of problem with Windows 8/Windows 10 since Microsoft has decided to abandon support for older 16-bit colour games. There are some work-arounds that can be done to get it running though.

Which game has better replay value in terms of replaying missions and mods?

To be honest I haven't followed Swat 4 much so I can't answer that. I don't know how much mods are available for it.

I gave up on it long ago.

Tribes Vengeance is now playable completely free:

http://tribesrevengeance.com/

kinda odd i don't know why sierra or activision would allow this? especially since SWAT 4 is in legal hell surrounding Irrational Games and 2K, although i think 2K owns the license since they own Irrational at the time though it was 2K Boston for a while surround System Shock shit

I prefer SWAT 3, as it features some better AI.