I need to add an extension to my patio aprox 4'x4'. It needs to be 4" thick to support my AC unit. I need to know how to do it.

Dig out the area were your pad will be about 8" deep. You then want to put down about 4" of 2b stone and tamp it down.
**Then using 2x4's make a form the length and width of the new pad. Use stakes hammered into the ground to secure the form boards. It is best to drill holes in the side of the existing patio and insert rebar that will stick out into the new concrete pour. (you will need a masonry drill bit and a hammer drill) This will connect the pads together and will help prevent the new pad from settling.
Then get wire mesh and put it inside the form. (this helps the new pad from cracking)
You can start pouring the concrete either from a truck or mixing sacrete bags. As you pour lift the mesh up into the concret a little bit.
Using a 2x4 longer than the form, screed the excess concrete. This will make the concrete level with the top of the forms. Any low spots will show when you start screeding.
As the concrete starts to set, use a trowel, regular or magnesium, to fishing the top of the concrete. Try not to over trowel and bring too much water to the surface. With an edger, edge along the inside edge of the form.
If you live in a warm area, as the concrete dries, you want to keep wetting it down a little so it does harden rapidly. If you live in a cold area, a concrete antifreeze can be added to prevent the concrete from freezing. Also puttting hay on the concrete helps.
**One final note, check the AC manufactures specs to see if the pad needs to be level or pitched.

A couple more points: Try to get below the disturbed dirt next to the existing patio and foundation of the house. Rounded stuff such as pea gravel or ''L'' stone takes less tamping then crushed stone.

A few years ago about this time of year I tried to buy concrete antifreeze at Lowe's. No got. As I walked out into the parking lot, another customer claimed to be in the business, and always used plain old automotive antifreeze. I have a degree in chemistry and thought the glycol should cure into the concrete much like water. I tried it, and the concrete I poured 6 years ago is still in good shape.

When I installed my A/C 10 years ago, it came with a plastic pad I put on leveled gravel. I did all the strong back stuff and then had a licensed technician come and silver solder the connections, purge, and fill with Freon.