I have a ceiling fan that stopped working. I have a 5 wire capaciter, 4,4,5 microfarad, I was wondering if I could just replace the capaciter with a single one for the high selection and install a wall fan speed selector.

You can, but you have to work out what value it needs to see for the proper high speed. It is a LOT easier just to replace it with the proper 5 wire capacitor.

Ceiling Fan Capactors - Troubleshooting and Resources - Ceiling Fans 'N More