He makes them entranced by his cock.

He waited a bit until the clone he had sent off dispelled, giving him the memories of what he had written, and grinned. First off, he had gotten the name of the innkeeper, and made sure that said innkeeper would not let anyone else into the woman's side, using whatever excuse was most believable, it mattered little what it was.

Then the clone had written, "Yue, Emiru, Nae and Sumire will immediately enter obedient trances where they can be programmed upon seeing the dick of Naruto Uzumaki. Any command given to them in this trance will be followed upon them waking. This will only work once so as not to make them freeze up during sex."

Upon getting confirmation that was written, Naruto hummed and entered the springs. Predictably, the four girls all looked, and shrieked. Naruto feigned confusion, and asked, "What? Isn't this the mixed bathing area?"

"No, it's the woman's bath!" Nae replied, though she didn't seem as pissed, seeing as it could be construed as an honest mistake.

"Ah, s-sorry," Naruto said with well faked sheepishness, and covered his eyes, "accidentally" releasing his towel, and causing it to drop, exposing his dick before any of the four could react. Though their first instinct was to cover their eyes, they instead found their gazes irresistibly drawn to his cock, their eyes blank.

"... hypnosis via looking at your dick Master?" Sakura asked, swaying her hips as she strode over to him.

"Yep," he grinned, looking over the enthralled girls.