MD is the way to go if you really only want 4 tracks, but it's getting dated now because hard disk recording is so cheap. Most HD recording kit does 8 tracks or more (though you often can't record on all of them at once), and the lower end of the digital multitrack recording market is served by the machines that use solid state memory. (e.g. Fostex MR8).
You may find a second hand MD multitracker for a good cheap price on eBay, as users upgrade to hard disk.
I have a Sony MDMX2 Mk2 There are others made by Tascam and Yamaha. I don't think any of them is available new now.