Form up with 2x4s, so you will get 3.5" ... pre-level the area with light shovel work to minimize ground disruption. Use RediCrete which has gravel in it and you will be fine ... a gravel bed is an added bonus. Just do your best to get the forms level. (Unlike the pic below) I have heard debates that the condenser MUST be perfectly level versus "Aw, BS, it can be anything close to level." I prefer the former because the unit lubricates best when level.
Another note ... set up your condensate drain so it does not undermine the pad. It can wash out and the unit will start to sink right where the condensate line drips off. Run the line away from the pad a bit.