Surprisingly, this did not bother me. Yes, of course it was clear that the smartest thing to do was go back to the ship and get more bodies, better trained bodies. They didn't because they didn't want the little girl to get raped, which is what would certainly happen if they delayed. The whole "bring me your younger daughter" bit was there to give the rescue some urgency.
As for how plausible it was, they live in a universe where enemies, even enemy soldiers, are bumbling incompetents (episode 1), where any average Navy seaman is a crack shot (episode 2), and where the Captain and his XO leave the ship on missions (all episodes so far). This thing about a Navy Commander leading ground missions is straight out of the original Star Trek. Before you ask "why did the Captain turn around?" you should ask "WTF was he doing there in the first place?"
Within that universe, I immediately understood why he thought 4 Navy guys were enough, strike that 3 since the ship's top-ranking NCO stayed behind to babysit the casualty. This is Kirk and Worf and a SEAL commander. They fully expected that they'd silently ambush the first three thugs they met in the dark, take their weapons, and pick everyone off one by one. Which is how it worked. As long as they're only encountering bad guys 4-5 at a time, they have them outnumbered. Which is how it was in every Star Trek, they were all masters of hand to hand, even Bones, if the script called for it.