Hi Amit,
NWDS 7.31 SP15 is the latest version available in marketplace .
For PO we should take NWDS version <=PO version .So for 7.4 SP08 ,7.31 SP13 is equivalent version .So opt for NWDS with version <=7.31 SP13.
FYI:Couple of our colleagues used NWDS version >PO version they have faced the problem while deploying NW BPM scenarios,but for couple of folks those who installed lower version of NWDS they could able to work comfortably.
Regards
Venkat