My husband and my best friend, who is also my child's God-Mother,(couple) are having a destination wedding. This wedding will cost us almost $4000 I was not asked to be in the bridal party, nor was he. We are wondering how much of a monetary gift we should now give them with the fact that we are just guests and we will be paying close to $4000 for the trip as well. I don't know what would be the appropriate amount,

You could just not go, and buy a typical gift.
Or go, and spend maybe $30 on a sugar bowl.
Your first sentence makes it sound like your husband is marrying your best friend. Who is getting married?

I can't imagine anyone assuming that all the guests can afford $4000 to travel to their wedding. I'd send my regrets.

Monetary wedding gift? Who decided that? It's tacky unless this is your child getting married.
You don't bring gifts with you to a destination wedding, and you have plenty of time afterwards to send one to the place they live.

Assuming paying 4000 to go to a wedding is nothing for you, I guess buy them a new car