absolutely, the most cost-effective answer, however do like the option of a lot of those effects that are available with, say the S4, as opposed to just using turntables.
that being said - with 2 x turntables + s4 i'm thinking timecoded vinyl is useless without a 'Scratch' unit - i.e. can you control the mp3s via the vinyl? and also, i notice on the back of the s4, there are inputs for turntables but no grounds - and the outputs - can that not go straight to an amp or does it have to go via a laptop/pc ?
again, apologies for the rookie questions, but then again, what do you expect - still listening to happy hardcore from 1992 on the old ipod classic. good times.