Low Tire pressure light

Our 2007 Nissan Altima’s Low Tire Pressure Light flashes for 1 minute when we first start it then stays on. I had the tire pressure checked and all tires were 4lbs low. I had the shop fill the tires to 32lbs then drove the car. No change. I went back to the shop and they checked the sensors and all were transmitting a signal Any ideas?