It really does sound like your mash thermometer is off a bit causing you to mash lower than you actually think you are. With a 157F mash temp (even on a standard ordinary bitter recipe) you shouldn't be finishing as low as 1.004. If you don't have anyone you can trust to calibrate your thermometer with, then here is a way around it which I have used in the past.
Try brewing a solid known recipe (ie from Jamil's Brewing Classic Styles) and using the mash temp in the recipe. If your FG finishes too high or too low than you can try making adjustments on your next batch using that information to help you get closer to your actual thermometer temperature. For instance, if you are brewing Jamils IPA and finish at 1.005 when his recipe calls for a 1.012 and you mashed at 149F like he calls for in the book, then you know that you should try mashing higher next time to hit a 149F mash rest. For the next time you try that recipe or even when you try mashing at 149F again, you may need to actually try a 154F rest instead and see where you end up. If you brew the same recipe a few times in a row you can quickly dial in your difference in mash temps using your thermometer and then you will know (more or less) how many degrees high or low your thermometer is running.
Hope that wasn't too confusing. Obviously when brewing these known recipes you need to practice proper pitching rates, aeration, and yeast/fermentation management to be as accurate as possible.
