Simply use \includepdfmerge instead of \includepdf.
It inserts pages of several external PDF documents. It should allow nup=2x2 as part of its options.
From the pdfpages manual, page 8:
\includepdfmerge[<key=val>]{<file-page-list>}
<key=val> -- A comma separated list of options using the
<key>=<value> syntax.
<file-page-list> -- <filename>[,<page spec>][,<file-page-list>]
A comma separated list of filenames and optional
<page spec> specifiers. A <page spec> can be everything
the option pages accepts. Leading and trailing spaces
of items in the list is stripped.
So if you want to have pages X till Y from A.pdf and pages Z till V from B.pdf in a 2x2 grid use:
\includepdfmerge[nup=2x2]{A,X-Y,B,Z-V}
e.g.:
\includepdfmerge[nup=2x2]{A,1-2,B,1-2}
Will first use pages 1 and 2 of A.pdf and then pages 1 and 2 from B.pdf in a 2x2 grid.
Guessing from your example code in your comment I guess you need the following:
\includepdf[nup=2x2,frame,landscape]{%
c:/Victor/Research/Presentations/Beamer-Presentations/Dam-Nonlinear.pdf,34-40,%
c:/Victor/Research/Pdf-Library/Saouma/ASCE-Cyclic-Joint-Model.pdf,1,%
c:/Victor/Research/Pdf-Library/Saouma/ASCE-Cyclic-Joint-Test.pdf,1%
}
(I broke it over several lines for readability, which is not required in your document.)