I am installing a 100 amp sub panel 12 circuit from my meter to portable buildings. Actual amps is about 70. I was told to use either #4 copper or #2 aluminum. For the 250' run. I will be running it on poles exposed.
One has suggested I use 2-2-2-4 aluminum for cost reasons. I am confused on the size of the wire. Is that #2 AWG or 2/0 AWG? What are the differences?What recommendations can anyone suggest?

You need to use a quadruplex aerial cable, has 3 - #2 aluminum insulated and 1- #4 bare that will act as the messenger cable to attach to each building using an insulator.

You are aware to use a ground rod at the building?

Thanks for your confirmation. Yes I am installing a ground rod at the building.