OK, so there is a Force = mg on each side of the pulley, 4g N for one mass and 3g N for the other, That gives you a resultant force of.......?
That force is accelerating BOTH masses, so use your resultant force with the total mass to find the acceleration. Common sense should tell you in which direction....
Now to find the tension, look at one of the masses, let's say the 4kg mass. Its weight is 4g. The resultant force on it you can calculate using F=ma but this time the m is just the 4kg and the a is the one you've just calculated.
Now you know the resultant force, you know the weight, the only other force acting on it is the Tension so you shouldbe able to deduce that.
Hope this helps.