How safe is XOR with multiple independent but non random keys?
How safe is the resulting code? Can I store it online?


Very unsafe. No matter how you come up with these keys (passphrases, really), it will have extreme biases. These biases are going to be enough to make some pretty good guesses about your plaintext. This is what makes RC4 insecure in the WEP protocol, and RC4's biases are comparatively minor.  

This also has the problem that passwords in general have, which is that you as a human can't accurately remember complex passwords. You'll have to either write them all down (in which case they can be discovered) or use some scheme which actually lowers their strength (your keys 1 through 3).


  Can security hold if 1 or 2 keys (but no more) are compromised?


For every key that is discovered, the biases of the other keys become more apparent. If 2 keys are discovered (particularly keys 2 and 4) it becomes pretty much a puzzle you find in a newspaper to solve.

My advice is to stick to known algorithms. If you need to encrypt something with a password, use something like bcrypt to expand your password into a high-entropy key, then use that key to encrypt with something like AES in a good mode like CTR.