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The 4125 is a single stage. The 5120 is a two stage. The 5120 will give you more pressure. The 4125 will provide more air. Single stage compressors use all cylinders to pump from atmospheric to final tank pressure. Two stage compressors use 1, or in the 5120's case, 2 cylinders to pump from atmosphere to an intermediate pressure, say 30 psi, then the air passes through an intercooler to cool the air before it is pumped into the second stage cylinder(s)where it is pumped up to final tank pressure. Single stage units will get up to pressure quicker, but the cut-out pressure will be lower. Two stage units will pump a bit slower, but reach a higher final pressure. Yes, you can adjust the pressure switch on the single stage compressor to whatever you want, but you will be overloading the motor if you crank up the pressure, and the pump will run very hot. Also, you can speed up a two stage compressor by putting a bigger flywheel on the motor, but again, you will be overloading the motor and the compressor will run hot. So it is up to you. If you need higher pressure (150psi or more) go with the 5120. If you need lots of air in the 100 to 125 psi range, go with the 4125. Sounds like you might be running a lot of sanders and the like, in which case I would lean towards the 4125. If you need lots of pressure for impacts and such, go for the 5120. The Quincy is the best reciprocating compressor you can buy. The IR's are pretty close in terms of durability. If you can get all three (Quincy and the 15Ts), you would have a very nice air system with lots or redundancy for maintenance and periods of high air demand. If the pump is on a tank, look for a small metal badge on the tank. It will say what year the tank was built. If the pump is original, it will have been built very near the same time. Look for YEAR, YR or YR BLT on the tag, followed by the full 4 digit year, or the last 2 digits of that year.
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