I believe aptX is supported. When connecting a pair of Phiaton PS 210 BTNC bluetooth earphones with aptX to my S4 GPE, I see the following log output from doing a logcat:
That would seem to show that the aptX codec is being used. The sound quality is also very good, though I can't honestly say if it's better than the sound I was getting when using the earphones with my Galaxy Nexus.