All depends if your 4 track has direct track outputs. If it does then simply plug each one into a 788 input.
Bit more tricky if you haven't but you could use the 4track's Pre fader AUX Sends if you have them. Two of the tracks would be recorded from the main LINE outputs (L & R). For the other two, use AUX Send 1/L and AUX Send 2/R setting the main faders to -inf.
If your 4track doenst have pre-fader AUXs then you can make use of the 788s track editing facilities. Record pairs of tracks from the 4tracks LINE OUT and then line them up using the MOVE>PASTE command.
