In this case, even if you customarily dip more than once, it would be fully permissible to dip only once. The bracha should be recited before the immersion.
Please check with your physician if on the night of scheduled immersion you could get your eyes wet momentarily. Explain that they will be closed underwater, but not shut tightly. If there is any special concern about infection, you can arrange in advance with the mikveh to be the first to immerse on a given night.
If your physician feels that even a brief immersion in the mikveh would be problematic four days after surgery, you will need to postpone mikveh until it is medically permissible. Discuss with your physician when this would be – we hope it will only be a brief delay. Once you have completed your shivah neki'im, you should change to colored underwear and do not need to do any additional bedikot before mikveh.
Please get back to us with any additional questions.
Refuah shleimah!