2 track or 4 track reel to reel deck?

For the same size tape and tape speed, a two track deck offers better signal to noise ratio than a 4 track deck.

If you're recording stereo, the 4 track gets better tape economy. Two of the four tracks record stereo in one tape direction. The tape is then flipped (or reversed, depending on the mechanism) to record stereo on the other two tracks.

If you're recording live music, a suitable 4 track deck will allow you to record a seperate signal on each of the four tracks for later mixdown to stereo or mono. Each track can be recorded independently while listening to the others, to allow building of complex arrangements via overdubs. Of course you can get recorders with many more than 4 tracks for this purpose. 8 track reel to reel is very common, and commercial studios can use 48 tracks or more.

Studios usually use 2 track to capture the stereo mixdown from multitrack recorders, for those still using analog.

What do you want to do with the tape deck?

You can only record 4 tracks as stated above, if the reel to reel is 4 Ch 4 trk...I dont see how you could do that with a consumer 2 ch, 4 trk reel to reel.

I have a vintage reel to reel that plays 2 track and 4 track, and it records in 2 track...the frequency response in 2 track is greater by a wide margin. There is a reason that commercially prerecorded reels are so much more expensive that 2 track.

It's been awhile, but I don't believe that you can play a pre-recorded commercially released reel to reel tape (4 track) on a 2 track machine. You can, however, play a 2 track tape on a 4 track machine.