I always read that when cooking a pot roast in a slow cooker, you should cook on low for 8 hours. If you don't have time, 2 hours of high and 4 hours of low is OK, or 4 hours on high; but the best result is 8 hours on low.  I recently got a new slow cooker and found it hits the safe temp (145F) after about four hours on low.  Unfortunately, I never checked the temp halfway through on the old one for comparison.  Should I take it out, or should I keep cooking for the remaining 4 hours? 
I highly recommend cooking pot roast (or any low-and-slow stew or braise) by checking the texture, rather than using time or temperature. Something like a steak can be cooked just to the desired temperature and served because there's very little chewy connective tissue to break down. Pot roasts are different. The thing that makes them taste so meaty, juicy, and tender is breaking down a bunch of collagen and connective tissue. That doesn't happen right when the roast reaches 145F; that's just the temperature where the roast is safe to eat. It needs to sit at that temperature (or warmer) for a while to give the connective tissues time to break down, so if you pull it out right at 145, it might still be very tough. (Collagen doesn't break down until 160, so I'd expect it to be pretty tough and dry at 145). How long that takes is highly variable, so you also can't use time to tell when it'll be done. I've had some finish in 2 hours and some in 4 (I do them in the oven, which is quicker). What you'll need to do is check the texture. Grab a couple of forks and try pulling the meat apart. If it easily pulls apart without much resistance, it's done. If you find yourself fighting to get it apart, keep cooking. You can also pull out a chunk and taste it, as long at it's passed that 145F mark so it's safe. If it's easy to chew and tastes tender, it's done; if it still tastes tough, leave it for longer. The good news is stews and braises reheat beautifully; they actually taste better reheated. So you don't need to time them exactly. Just shut them off when they're done and reheat when you're ready to serve. 