Depending on the motherboard but most will still work with only a 20pin connector, so I'd check to see if it works with just a 20pin first.
If not a 20pin to 24pin adaptor should be fine eg
http://www.lindy.co.uk/atx-psu-to-ea...ble/33222.html
After a quick look, no I cannot find any +4pin adaptors, only way to do it would be to make one up yourself.
sata to +4pin would be better than molex to +4pin as the +4pin is one earth, one 3.3v, one 5v and one 12v wire
molex lacks the 3.3v, it's also the reason that some drives don't work with a molex to sata adaptor