I use eagle-cad 6.0. when I print the circuit layout onto PDF, it prints one circuit per page. However, my board has a small size and 4 layout could fit a A4 page without any problem. Could anyone tell me how I can print 4 copies of the layout onta a A4 page without changing the actual dimensions?
As @mattyoung suggests, you want to panelize. Eagle doesn't panelize. However there is an incredible piece of software which panelizes gerber files fantastically well. It's called gerbmerge, and it's free. It runs on Windows and Linux (and OSX, and anything that can run unixy code).
I highly recommend it for this kind of work.