I would definitely check the corner monuments first. Know where the property line is.
The holes are at least 6", not 4", which is the post width. They only need to be 2' deep.
I use a guy who runs a bobcat drilling company.
We usually do 300 holes, and run around 300' of trench every year. I'd say the total cost for around 3 hours of work runs to less than $400, which includes travel to and from the shop.
