What's the length of the loan you're thinking of? If his business situation is really sound and this is a short term problem, it should be a short term loan, maybe a couple of years. If he's not planning on paying it back quickly, perhaps his business situation isn't as good as you think.
Really, what you're giving him is $2,000 per year in interest savings (shrinking steadily, unless there's a balloon payment). In exchange, you're taking on a $50,000 liability for 4% interest.
Since the money doesn't seem particularly significant to you, I'd be more concerned about losing the friendship than losing the money. If I were in your shoes, and he were really a good friend, I'd suggest that he keep the loan, but I'd offer to just give him the difference in interest payments, i.e. I'd give him $5K to make the interest payments as he pays off the loan over three years. If he can't service his loan with the bank, you haven't lost much money and you're still friends. If things go well, he'll probably insist on paying you back at the end of the three years, and I'd take the money. But I wouldn't ask for, or take, any payments in the meantime, and I wouldn't even want to discuss it.
YMMV