That's too bad... I know I wouldn't have waited 4 months!!! It does sound like a rare complication due to anal sex. I don't know how large the risk is of reopening the fissure once it's cured, but here's my advice on anal sex:
Most people don't just "dive in"! If you truly want to have anal sex comfortably and
safely, you need to start a gentle stretching program weeks before you go for it. This can be done easiest with a series of anal plugs that slowly get larger in gauge. You start with a little one the size of your finger and wear it for a few hours a day to become comfortable. Because you're slowly and gently stretching the sphincter, you are not causing damage. You finish with a plug that is about as wide as your partner's penis. Once you are comfortable inserting and removing that plug, you are ready for the real thing. Use a lot of lube, and everything should go just fine.