It depends on what kind of heat pad you're using. I've used the zoomed heat pads as well as homemade flexwatt undertank heaters and heat rope. The flexwatt was extremely weak and never really brought the tank up to temp, but the rope and the zoomed pads worked really well, to the point where if I wasn't using a thermostat they might have done some damage.

If your hardware is the problem, you'll just need to switch it out for something stronger. It's possible that it's not getting any power for some reason, as well. Put your hand on it. Does it seem warm at all? 4 hours seems like a long time for there to be no difference. Make sure the probe for the thermostat is against the floor directly over the heat pad, too.

Also, those rubbermaid tubs are really bad at holding heat in the first place. It might just be radiating it away into the environment, especially if you're opening up the lid a lot to test the temp. You could go and buy one of those metallic emergency blankets. They insulate extremely well. Cut it up and wrap it around the outer walls and the bottom of the enclosure (heat pad on the inside, obviously) to help hold the heat inside.