4 months post hair transplant, very little visible results. Is my hair transplant a failure? (photos)

After the operation the grafts will be entering the ‘resting (telogen) phase’ but will remain healthy and will enter the growth phase (Anagen) and be active in time. You can expect to see new hair growth around four months after the operation, but it will not be that visible at first. They will also grow at different rates and mature slowly for up to a year or 18 months before you see the final outcome of your hair transplant operation.
During the re-growth period, your hair will initially grow thin, then over the course of a year they will become thicker and darker.

You need to give yourself more time. usually, my patients have 80% of hair at 4 months, I am using a very "nutritious" and caring regimen after surgery. Under usual conditions, you mill see good results at 6 months, fuller at 8 and full results at 12 months.
So, give it time!

At 4 months, you're just entering the Growing phase. Normally only 10-15% of transplanted hair grow in this month. It's not surprising why it looks so thinning now. Keep in mind that hair transplant takes 12-18 months to fully grow. It's way too early to say it's good or bad result now. In my opinion, you progress isn't bad.