Yes, you would still need to use 2 molex connectors. The MB994SP-4S is designed to use two molex connectors because each connector powers up two drives. If you only have one molex connected, only two drives would work. It is recommended to use two molex connectors so that the unit will perform properly.
You could get a SATA 15pin Male to 4pin Molex Male converter if you have more 15pin connectors:
Please let us know if you have any more questions or concerns