My shower/tub take a few minutes to get warm, then we run out of hot water very soon after. I can not make he water hotter, why. It is much hotter everywhere else in the house, even sink next to itgets hotter. In the summer it is not as bad, I think because the water coming into the house is warmer and does not take as long to heat. I have been told it is something to do with mixer in the system. I have a hot water heating system that is only 4 yrs old.

You need to access the thermostat and make the water hotter that way. In my home the second bathroom is so far away from the hot water heater that only until I raised the temperature of the hot water heater about 10 degrees the hot water was not very hot in that shower. Now since raising the temp I have no complaints from my son who uses that bathroom shower about not enough hot water.

When you do raise the temp of the water you must remember to warn everyone in the household that the hot water will be much, much hotter and for them not to scald themselves when using the other bathroom and sinks.

That's the beauty of it though, not telling them

If you run out quickly it could be a bad element if it is electric, a thermostat that is set too low or a broken dip tube.