Man are you aware you can fit a 400C laser on a V4 ?
It's easier to get and the odds of finding a good 400C is way bigger than a 400B because they stopped making 400Bs in 2003...
And you say it might be the mother board... I'd say you have to check the edges of the spindle motor flat cable, the laser flat cable and checking the fuses with a multimeter isn't a bad idea at all.
If you don't remove the solder blobs, as klik said, you will get no laser (but I think it would not spin at all without detecting that a disc is in the tray)
No laser = mobo thinking there's no disc in the tray.
Post more info so we can have a idea of what's going on
SCPH-10000_GH-001 SCPH-15000_GH-003 SCPH-18000_GH-008 SCPH-30001_GH-005 SCPH-30000_GH-016(V4) SCPH-30001_GH-010(V4)
2xSCPH-10190, 2xSCPH-10350, 2xSCPH-10280
"**** j0 hackers!"
-Sjeep (As seen on TOXIC OS ELF...)