You talking about the first vegas? If you are then go for it if you've never played it before. Although if you are going for a team leader style special forces game take a look at an old classic SWAT 4 (2005) and the expansion SWAT 4 Sketchov Syndicate (2006). Rainbow Six just reminded me of that game whilst not being better than it. Can't really comment on Quake 4, sorry.
EDIT - Just had a quick look and it seems most stores don't stock this game anymore however, you can get it
here for just over a £5er including postage!
One guy's review includes this:
Sometimes this game gets compared to Rainbow Six, but it's actually nothing like it, Rainbow six is shoot first and ask questions later and run through the map in a linear path kind of game, with SWAT you take whatever path you see best, and you have to follow Police procedure, you cannot shoot an armed suspect unless he is about to fire upon you, your team or a hostage/civilian, that is part of what makes this game great, if an armed suspect appears, all you can do is yell at him to comply and put down his weapon, if he wants to run away there's not a thing you can do to stop him immediately, suspects therefore have been programmed to be very unpredictable, some will give in instantly and drop their weapons, others will open fire, or some will run away, even hostages can be unpredictable, some will obey orders and drop to their knees hands in the air, others will refuse, again there's nothing you can do with your firearms to make them comply, this is where further skill comes in, you can use pepper spay and squirt it into their eyes, or stun them with a taser gun, or throw a cannister of CS gas near them.
