Let's say I have four computers, each mining bitcoins at 5 Mhash/s. Is this the same as mining bitcoins on one computer with a speed of 20 Mhash/s?
Yes, the hashrate adds up.
But GPU mining ended in 2013. The tiny bitdust you will earn will never reach a mining pool's minimum cash out limit. So you will never see any bitcoin. This will be a pure loss consisting of spent electricity and worn-out hardware.
You could buy a USB stick for mining, for 10 USD. It would mine at a loss due to electricity costs. Even if you have free electricity it would never earn back the 10 USD. So this would also be a loss.
The only way to mine now is with modern stand-alone miners. They have a much higher hashrate and efficiency and they use a tiny built-in computer instead of connecting to your PC. You'll want to get one that runs 1 TH/s (1000 GH/s) or faster. You will not want to pay more than 0.30 USD per GH/s for a miner with power supply. You will need cheap electricity.