Two blocks are connected by a light string that passes over a massless, frictionless pulley as shown. The coefficient of friction between the 4kg block and the surface is 0.4. Find the acceleration of the system and the tension in the string

You didn't post a picture, but that's OK. I assume the problem is a block on a table connected by a string over a pulley to a second block that is hanging over the edge, correct? Write out equations for the sum of forces = ma for each block, and you'll have two equations with two unknowns (tension and acceleration) which you can solve using a standard algebra technique. Try it, and post back with what you get for results.