Trying to merge two files with emacs... need help!

[Gary Z]

I'm new to emacs and I'm trying to use it to merge two files into one.
Some time ago, I watched a co-worker step through a file and
selectively merge changes into it from another file. Unfortunately, I
don't recall how he did it! Currently, I'm opening a file with emacs,
selecting diff (2 files) from the menu, specifying the current file
and the file I want to merge into the current file. I hit the space
bar and the first difference is displayed. I am able to accept what is
in buffer a or buffer b, but I can't figure out how to combine both of
them. The buffers look something like this:

Buffer A
here is the first line.
here is the second line.

Buffer B
here is the third line.
here is the fourth line.

The result that I want is:

here is the first line.
here is the second line.
here is the third line.
here is the fourth line.

Any ideas on how I can do this? I guess it's tricky because emacs
views it as a conflict, but I want to combine everything, not choose
one or the other.

Thanks,
Gary