I'm assuming you mean analogue tape & 4 Track isn't the best, that's fer sure. All you can do is record and mix everything as hot as possible, mute unplayed tracks, EQ everything so that each instrument makes it's own little pocket in the mix, both in frequency and panning. Possibly use a little compression to get the mix even hotter on mixdown & mastering.
Other than that, you're limited by the tape itself. You'll always be fighting against the signal to noise ratio and probably putting up with noise reduction as well.
Just a suggestion but dump the 4 track altogether and move into software based recording. Not just for the quality, but for the ease of use and getting out of linear recording. Nothing's better than being able to drag and drop tracks around, add in a dozen effects, play keyboards digitally or use amp emulation software without going analogue at all. Plus, depending on your hardware, you'll be capable of getting 100 or more tracks to play with.