The only way you could over carbonate the beer the way you did it would be if the temp was colder than you thought or the regulator is wrong. It could be a problem with a dip tube or poppet, or the line.
Still, the way you tested it is flawed.
I vented my keg last night and dialed down the pressure to 4psi and the beer seems to be serving pretty good today
By venting the keg you released some pressure, but overnight the gas will come out of the beer and you will have pressure in the keg again. You set the regulator to 4 psi, but that is not indicative of the pressure in the keg, just in the regulator. It's likely that all you did was remove some of the pressure in the keg, but it is probably still above 10 psi depending on how much beer is in it.
My advice is to vent it and begin pouring immediately. It shouldn't pour at all without pressure, so then you start adding pressure until you like the pour. As it sits though, more gas will come out of solution so even if you like the pour at 8 psi you might not like it tomorrow if enough gas comes out of the beer and raises the pressure in the keg above that.
While you are doing it look at the lines - where does the foam start? If it is foaming coming out of the keg even at very low pressures then I'd lean toward the dip tube/poppet as the problem. If it is liquid in the line but foaming at or partway to the tap, it is most likely that there is too much pressure at the tap, meaning your line diameter/length isn't restrictive enough.
Once you know at what psi you like the pour you can vent the keg repeatedly to get it down and stable at that psi, or just extend your lines to make up for it.