If it's only 2 bits, you're assuming the attacker already knows that the formula is "Using someone else's information" and that there are only 4 possible people whose information you would use.
Even knowing that you're using a formula is a bit of information. The type of formula is potentially thousands of bits of information. An attacker doesn't know whether it's a cipher, or a code, or something more complex, and only then can they begin figuring out the parameters to that formula.
Pretty sure lots of people use relatives' info. Very, very few use ciphers in their head.
Friend used to have a car with a keycode door lock. He just used 5555 or whatever. I suggested he use the address where the car was parked, or some hash of that. Wouldn't have to remember it! And it would vary some at least.
Well, sure, 8 bits of entropy isn't going to help you much if your password is "password". Those bits only provide the opportunity for randomness. At the end of the day you still have to apply that entropy effectively by picking something that can't be guessed easily. The point is that there are opportunities for people savvy enough to recognize them.
Even knowing that you're using a formula is a bit of information. The type of formula is potentially thousands of bits of information. An attacker doesn't know whether it's a cipher, or a code, or something more complex, and only then can they begin figuring out the parameters to that formula.