4 PSI? I'd have to pump my tires up every day. I'm too lazy for that. I pump them up to about 90, and don't bother pumping them up again until they're down to around 50 or 60, which is still pretty decent for the 32s that I run.

[edit] actually though, that isn't a bad idea for cars. On our cars I check tire pressure every couple of weeks, and it takes months to lose 4 PSI, except for one tire that goes down about 5 PSI in 2 weeks and I'm too lazy/cheap to have it remounted to fix it. It's probably the rim anyway and I'm not gonna replace a $500 rim on a 12 year old car.