Your sentence is okay, but in natural speech we usually don't describe people as "regretful." Instead we use the verb form, and say that the person "regrets" something. If you've already explained what he did wrong, I think the most natural sentence would be:
"He regretted it four months later."
"Four months later, he regretted it."
A couple of examples:
"In March, I decided to buy a new car. I regretted it four months later, when I realized I couldn't afford the payments."
"He broke up with her in a fit of anger. Four months later, he regretted it, but when he tried to call her, he found out that she had already found a new boyfriend."