I own an FW04 4 stroke along with a few others on this board. You can buy the kit version but its pricey. I think you are able to still get it. The FW04 hasn't been discontinued yet.
No one else that I know of makes a stock production kit with a 4 stroke. Of course, companies like New Era Models have made 4 stroke conversion kits for various monster and stadium trucks. Someone else made a conversion kit for an HPI SNRS4.
Is it competitive? Yes, it handles fairly decent well and the 4 stroke is great for small tracks. Just make sure theres a class you can race it in. There are many options for the FW04. I recomend getting a 2 speed for it.