A 4 inch slab is a standard thickness for non commercial grade structures.
Pouring fresh concrete over existing concrete is not unusual. The company I work with does it frequently on remodel jobs when slabs are out of level.. You have to rough up the existing slab a bit and coat it with a commercial concrete adhesive, then apply the new concrete. We have used "Ardex" with pea gravel mixed in but our thicknesses have been less than 2 inches. This is quite expensive though. Ardex is a self leveling concrete powder and is not as structurally strong as actual concrete that has rocks and sand mixed in with it.
this.**** And this*** "A good grated drain LENGTHWISE in front of your garage will take car of the water and should be affordable " " you can pour concrete at 2" with bonding agents and add plasticizers . it needs to be rough as stated and you can also get a higher psi [4000] strength concrete. i don't know the cost in your area but about 5-6 yards will be all that you need. [i would guess $1200.00 plus additives and add finishing labor ] if its settling and cracked, you can use a jack hammer or a skid steer with a hammer to break and settle the current slab to prevent future cracking!
