the blue wire sounds like the power antenna wire, and if you don't have a power antenna then don't worry, it could also go to an amp. The other three are kind of a toss up, I've installed more stereos in my friends cars over the years than I care to remember and they are all differnt. I would guess they could be your continous power wires, so they would go to a full power spot in your fuse block, so they can keep power in the stereo so your presents and clock stay correct. I havr no idea what the grey wire is for, but I can guess it might be for the factory Dim on the radio...I think you can jmust ignore it because there is no dim function on my aftermarket stereos. I hope this helps, sometimes the company just adds extra wires for more speakers and amps so they could all do nothing.
Good luck
-Josh