Hello Richard,
It sounds like you are having 4-weekly dose Zoladex implants, which is a bit unusual. It's more usual for men to have 12-weekly dose (10.8mg), possibly starting with just a single 4-weekly dose. It doesn't matter, but you are having the implants more often than if you were to have the 12-weekly dose.
You will need to ask your oncologist how long they want you on Zoladex. This can range from 1-3 years if the RT is your primary treatment with 3-6 months before treatment, and the rest afterwards, but is sometimes less if the radiotherapy is a salvage treatment after a failed prostatectomy.