The ideal combination for recording four microphone sources would be to have a Focusrite Platinum TwinTrak Pro with optional digital card connected to the SPDIF
input on the Saffire, providing a full complement of Focusrite front-end. (Any Platinum with digital card would be fine though.) Novation's X-Station has a SPDIF
Out, which would also be a good combination with the Saffire; a backing track could be played on the keyboard and recorded whilst a singer and guitarist are recorded simultaneously. Alternatively, the Focusrite owner with no digital i/o could go directly into the Line input(s) to convert to digital, gain access to additional signal processing and then send to the recording platform.