These kind of threads are the BEST! Its so much fun spending someone else's money :LOL2:
Like some others have mentioned, are you wanting to spend the entire $4k on an engine, or would you consider getting a used engine and saving a couple grand?
If you want to spend it all, a new 25HP four stroke would be my recommendation. Honda, Tohatsu, Yamaha, Nissan, Mercury etc.... all good motors. Decide based on which one has a dealership close to you that provides excellent service levels. 2012 leftovers are on sale everywhere right now.
If you don't need to spend it all, you can probably find a mid to late 90s 25HP two stroke Johnson or Mercury for about $1,000. If you shop wisely, you can find one that runs like a top and you'll still have $3k in your pocket.
If you decide to buy used, look for a boat/motor/trailer combo on Craigslist. The whole combo usually sells for about what the motor would sell for by itself. Keep the motor and sell the boat/trailer to recover most if not all of your cost.