Low pressure after water main pipe replacement
The tank has a bladder inside the bladder holds the water on the outside of the bladder( inside the tank is filled with air) . With the tank empty of water we normally pressurize the air to a few pounds less than the pump turn on pressure. So if your pressure switch is set for 40-60 it turns on at 40 psi and when it reaches 60 it turns off in this case we Would pressurize the tank 36-38 psi. This pressure pushes most of the water out of the tank with out the pump needing to start until the pressure drops to 40 psi. If the bladder had a leak and was not holding air the pump would short cycle rapidly and your pressure at your faucet has low flow. To check this turn your pump off drain the water out, using a tire pressure gauge measure the air pressure in the tank there is a small Schrader valve on the top of your tank.

if the pressure is below your cut in or pump on pressure by more than 5 psi it needs to be filled. A standard air compressor or bicycle pump can be used to pump it up. It may have a leak or got bumped when the pipe was replaced if it holds everything should be back to normal. Some manufacturers do make replacement bladders but seeing that they are not much cheaper than a new tank and a new tank will have a warranty I usually go new tank but have purchased bladders when they were reasonably priced. If the tank is holding its air charge the next thing is the pressure switch, I have had them freeze and not quite work correctly so it had to be replaced. If you don’t have a pressure gauge on your water line after the tank, it is a good idea to have one this way you can see when your pump is cycling and the pressure values.   This should help you troubleshoot the system. Your plumber should have checked the air charge especially since the tank is only 4 years old, bladder based tanks at normal pressures have lasted over 20 and some closer to 30 years so I would check it out prior to replacing.