Why won't my A/C cool my house

I have a 4 ton A/C unit and my house is 1,800 sq ft so it's my understanding that this unit is big enough to cool my house. I do live in south Texas and the heat, for instance, today is 90 degrees with a heat index of 100. Humidity is 61% and Dew Point is 75%. I have thermostat set at lowest setting yet it is right now (at 4:14 pm) 76 degrees in my house! Plus all my closets smell musty and my vents are dripping. Add to that the problem aside from the A/C not cooling is that when I set the thermostat at a set temp, say 70, and if the temp was to reach that temp (ha) then it kicks off; then when it tries to restart when the temp goes up to 71, it trips the breaker! So, I have to leave the thermostat down at the lowest setting and just let it run 24/7. Help!

To recap:

Question one - shouldn't a 4 ton A/C unit cool a 1,800 sq ft house?
Question two - why won't A/C come back on without tripping breaker after reaching set temp.

Thanks for any and all help for this old lady!:gah:

Extra info: House is brick, concrete floor, built in 1967, all windows have hurricane shutters which are closed to block the sun. Lots of shade trees.