Hi pg72 and welcome,
SBRT actually has the
least amount of side effects of any therapy other than active surveillance. You can read the latest about
it on the following links and in the two links in my signature.
Quality of life after three different first-line radiation therapies for localized disease: Part 1High Dose Rate (HDR) brachytherapy was not included in that study, but it has very similar outcomes to SBRT because it is radiologically identical.
Seeds did have the most side effects of the three radiation therapies in that study. However, with your GS4+3, I think a lot of seed doctors would prefer to give you a combination of seeds and external beam radiation.