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Pork loins usually take about 30 minutes per pound to get to an IT of 145 so a four pound loin would take 2 hours. You are actually better off to cook to 140 then pull and wrap...the last 5 degrees will happen during the rest.
If you are worried about your temp do a calibration check on your temp probe by placing in an ice bath - should be about 32 F - and boiling water - should be about 212 F.
Remember BG.. leaner meats cook faster. Less fat and collagen to break down and cool the meat as it does so. Also less moisture so you definitely want to watch your temps. PG says it right when he says to wrap....especially on leaner meats where the temps will rise and meat won't dry when you wrap.