Wiring a Pole Barn
You CANNOT use 10/3 for a service. The #4cu & #2al is just that. #4 gauge copper or #2 gauge aluminum conductors. As opposed to #10.
I'm still not sure what your service box entails. Is this just a meter pan, or is there a disconnect breaker out there? This is one of the main things we need to clear up.
Can you possible post a pic of the service post the POCO provided?
To be honest, if you are considering #10 to feed the whole pole bard, and do not know what the conductors I was referring to are, I really feel you would be better off just letting an electrician doing the whole thing. Either that or have him come out and explain exactly the things you can do prior to his coming to complete the job.
Without knowing exactly what you have and/or seeing it, it is hard to get a grip on what you could possibly do yourself.
Sometimes I feel like if I answer any more questions it is like someone trying to climb over a fence to jump off a bridge and me giving them a boost.