Good day, I am trying to find the ship manifest for my grandfather Benjamin/Ben Kaplan (previously known as KAPCHITZ). His petition for naturalization shows that he left Rotterdam, Holland going to New York, New York in April 1904 on the Potsdam. I have looked up the ship site and went through all of the ships that came in during 1904, and I have been unable to locate him on any manifest. He would’ve been 19 years old at the time. Can anybody help? Thank you.
Donna Kanlan

Donna,

This may be him.  Nochem Kapschitz, born about 1882.  From Minsk.
Going to his cousin Mosche MABLIS (surname very hard to decipher on manifest)  40 Canal St., NYC.

Name:	Nochem Kapschitz
Gender:	Male
Ethnicity/ Nationality:	Hebrew
Age:	22
Birth Date:	abt 1882
Departure Port:	Hamburg
Arrival Date:	27 Mar 1904
Arrival Port:	New York, New York, USA
Ship Name:	Pennsylvania

Good luck in your search,

Sherri Bobish

Searching:
RATOWSKY / CHAIMSON (Ariogala, Lith.); LEFFENFELD / FINK / KALTER (Daliowa & Jasliska, Pol.)
BOJDA / BLEIWEISS (Tarnow & Tarnobrzeg, Pol.); WALTZMAN / WALZMAN (Ustrzyki Dolne, Pol.)
LEVY (Tyrawa Woloska, Pol.); SOLON / SOLAN / SOKOLSKY (Grodek, Bialystok, Pol.)
BOBISH / BLUMENKRANZ / APPEL / WEINER / ROSENBERG (Vysoko-Litovsk, Brest, Biala Podlaska)

Donna,
I believe this is your Benjamin. Berl Kapzcic traveling with Jankel Kapzcic, line 10 and 11. 
https://www.familysearch.org/ark:/61903/1:1:JF17-92L.
Lee Goodman
Vermilion, Ohio

Donna,

The cousin's name that looked like Mosche MABLIS on the manifest is MABLIN.  On another page of the manifest (the detained alien page) it noted that the cousin's address was 128 Division.  A search of city directories found Morris MABLIN at that address in 1906.

NAME:	Morris Mablin
RESIDENCE YEAR:	1906
STREET ADDRESS:	128 Division
RESIDENCE PLACE:	New York, New York, USA
PUBLICATION TITLE:	New York, New York, City Directory, 1906

Hope this helps,

Sherri Bobish

I think you've got it. Just a searching hint for others.  Using ancestry this manifest is the top hit with first name b*, surname k*, date of entry 1904, birth 1884, and keyword potsdam.all of which the OP had available. On family search you do not have the option of entering Potsdam so the correct manifest is further down. But using name k*p* gets it into the top 10
Paul Chirlin
Florida