Question About Military Rankings & 4th Marine Regiment

I have a general question about rankings.

It's my understanding that the 4th Marine Regiment included Navy personnel that previously fought in Bataan.

If someone was a Lieutenant in the Navy but then became a part of the 4th Marines, did they retain their rank (Lieutenant) from the Navy or did this change to a comparable Marine rank? And if it changed, what rank would that be?

Appreciate any help. Thanks!

 

Either way it's an O-3, the Marine Corps is a sea service and as such is familiar with naval ranks. His rank wouldn't change because he was filling an unusual billet. A Navy Lieutenant is the same as a Marine Corps Captain, a Marine Corps Major same as a Navy Lt. Commander, a Marine Corps 1st Lieutenant (Navy Lt. J.G.) or 2d Lieutenant (Navy Ensign).
Depending upon his experience he may or may not have been used in a Lieutenant/Captains billet. Say he was a gunnery officer, he might very well have been placed in command of AA, DP or anti-ship guns. If he was in the Provisional 4th Battalion, here's what he would have been doing per Hyperwar:

"The sailors from Mariveles, mostly crewmen from the now-scuttled Canopus, were kept together and formed into a new 275-man reserve battalion for the regiment, the 4th Battalion, 4th Marines.8 Not only was the designation of 4/4 unusual, but so was its makeup and its personnel. Only six Marines served in the battalion: its commander, Major Francis H. Williams, and five NCOs. The staff, company commanders, and platoon leaders were drawn from the nine Army and 18 Navy officers assigned to assist Williams.9 The four rifle companies were designated Q, R, S, and T, the highest lettered companies the men had ever heard of. Another boast of the bluejackets turned Marines was that they were "the highest paid battalion in the world, as most of the men were petty officers of the upper pay grades."

 

Thank you so much! This is extremely helpful. Appreciate the information.