I'd pick up both, since MK4 is a really underrated title. MK4 may be a little more since it came back more recently, but less than the $5/6 it was stuck at for a while. Both the first 3 games and now MK4 sell really cheap on sale (maybe like a $1.25 each). GOG Galaxy also sells MK4 now too, which only just came back a year or two ago. Then you need to install those files into MK2 directory and change the audio in-game to Ultrasound rather than standard SoundBlaster. MK2 has midi music, but I use the pro patches lite version of Gravis Ultrasound (which you can find online). MK1 and 3 have the same CD quality music you'd have heard in the arcade version. But play around till you find one you like). It's not the same as the 2011 version that had the arcade quality graphics rather, it's the DOS versions, however the graphics are only very slightly downgraded (lower resolution images, but nothing missing like on consoles), and I can walk you through the setup to improve the quality if you like (hint, set upscaler to 2xsai, I find that to be the sweet spot for MK1 and 2. I got MK1 + MK2 + MK3 through GOG Galaxy.
