Hi, I was playing around with a Mikrotik at home and upgraded the firmware to version 4 (I have a level 4 license). I left it alone for a few days and now I can't get it to work. How should I go about reinstalling the firmware and keeping my old license? What cables/software will I need? Thanks to anyone who tries to help me, I appreciate it!

I think you've to update your license key. Remote your router using winbox. Enter menu System --> License --> Then click Update License Key Button. Make sure your computer that you use to remote router connected to internet. If update key successfully, router will reboot.
Or you can read this article : http://wiki.mikrotik.com/wiki/Manual:Li ... bol_SoftID