Q. What is it supposed to be on the show?

A. The entrance to the W 4 Street Subway Station. 

 
Q. Where is it supposed to be on the show?

A.  In New York City's Greenwich Village.

Q. When did we see it on the show?

A. In Episode 9 of Season One of Resurrection.  
 
After the scary run-in with Leon Prater at the steakhouse restaurant, and after Dexter & Harrison hurry down the street to the next corner, they reach this subway entrance. where they go down to the underground station and board a subway train.
 
What they don't immediately know is that Angel Batista is following them. But when Angel gets on the same subway car as the duo, Dexter waits until the doors are about to close, and then quickly exits the car as the doors close behind him, leaving Angel stuck on the departing subway train.

Q. What is it actually in real life?

A. An actual entrance to a subway station, just not W 4th St.  

Q. Where can I find it in real life?

A. This subway entrance is actually located at the southwest corner of Franklin Street and Varick Street, in the Tribeca neighborhood

It is not thereal 4th Street station. The signs are fake, as have been all of the subway signs we've seen in this series.
 
This is actually The Franklin Street station.
 
The GPS coordinates are approximately 40°43'08.3"N 74°00'25.6"WN
 
The orange/tan  building seen across the street, behind Dexter & Harrison in the as they are about to enter the subway, is located at 140 Franklin St. 
 
But take note that There are two Franklin St entrances, located across the street from each other. This isn't the same Franklin Street station that Dexter comes up from after chasing the Dark Passenger killer. That one is located right across Varick street, only about 120 feet away to the east, on a small traffic  island just off the east side of Varick Street. This one, on the other hand, is on the west side of Varick street.

They used both of these Franklin St station entrances in the first season, putting different fake signs on them. Neither one of them is supposed to be the Franklin Street station.  This one is supposed to be the W 4th Street station, and the earlier one was supposed to be the York St station.   
 
And  as far as I know, neither scene used the actual Franklin Street underground station, they just used the street-level entrances.
 
Instead, they shot these underground tunnel scenes down in the Bowery station, a mostly abandoned station which is located at the intersection of Bowery & Delancey Street, on the Lower East Side of Manhattan. (Read more about it on the page about the fake York Street Station. )
They used two different stairways down in the same Bowery station for the two scenes, so they wouldn't look exactly alike. But those two stairways are actually only about 30 feet apart from each other, down in the same station.

So, where is the actual W 4 St Station?
 
The real-life “West 4th Street–Washington Square station” is located  at the northeast corner of  6th Ave  and  W 3rd St, in Greenwich Village. That's about a mile northeast of this Franklin St station. 
 
( One obvious difference: the iron support columns in the real 4th Street Station are painted green, not red. )

These subway scenes are just two of many scenes they shot in the Tribeca neighborhood this season, ranging from Mia's apartment to the steakhouse restaurant.  

 
Here is a matching StreetView of the Franklin St subway entrance: 
 

 
Here is an aerial photo of the location.  And here is a map link.