Are the turntables connected through the SL4 or DJM-250?
If they are run through the SL4, through Serato, then just use the inbuilt recorder in Serato.
If they are connected through the mixer (which I assume they are), I would run the RCA outs from the mixer into an input on your SL4. I would then use Garageband to record the mix (its free). Just make sure that you don’t distort the inputs on the SL4 from the output of your mixer – but don’t have it too quiet either.. A signal level that peaks at about 80-90% of capacity should be about right.. 🙂
That should work.. 🙂
bob6397