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, and that'll net you around 0.01 USD worth of BTC per year (as of January 2016), assuming your power is free.
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.
Yes. More generally though:
x mining machines with an average hashrate of y hashes per second will have the same chance of finding an arbitrarily low blockhash in a period of time as one mining machine with a hashrate of xy hashes per second in the same length of time.