Free program to insert pages into a PDF document?

Let's say I have a 10 page document in PDF format. I also have a single page as another PDF.

I want to remove page 4 of 10 and insert the single page in its place. I used to have a program to do this but can't remember what it was.

Any suggestions? It has to be free.
Update: That would be great but I need a program with a free version
Update 2: I'm not asking for cracked versions of pay-for programs, I'm asking for the free versions of the programs. Like AVG do a free but basic version of their software.
4 answers 4