He chose to re-up for another four years of military service because he felt it was important to serve his country.
An example of re-up is when you take your oath and enlist against after you have served a tour of duty and are eligible for discharge from the army.
intransitive verb-·upped′, -·up′ping
Origin of re-upre- + (sign) up (see phrase under sign)
intransitive verbre-upped, re-up·ping, re-ups Informal
- To enlist again for military service.
- To sign a renewed contract for employment or service.
(third-person singular simple present re-ups, present participle re-upping, simple past and past participle re-upped)
Possibly blend of re-enlist and sign up