Yeah, I agree with Delta. The bigger disadvantage with the 4wt will probably be just that it’s tougher/more work to throw larger Smallmouth flies with. Yours is relatively long though at 8’0, so you should still be able to get some leverage to the fly. The rod should handle the fish fine I think though. A big Smallie is gonna be a handful no matter what rod you’re using, but I think the risk of it breaking your rod is very minimal. The only thing I could think of putting your rod in jeopardy is a 4-5+ pounder getting downstream of you in relatively swift water and then you trying to muscle him back with the rod against the current. Bottom line…just don’t do that and you should be fine.
My Smallie rod is a 5wt, but sometimes I end up on a warm water creek after trout fishing and all I have is one of my 4wt rods. I’ve caught 15+ inchers on my 4wts just fine…put them on the reel and use the drag to play them out. If you hook into a big one just know you’re fighting the fish on a light weight rod, and adjust accordingly…let em’ run, don’t horse them in, and although it will probably be rare/almost never, know when ya gotta let one break you off (as Delta mentioned). Fish with 8lb (give or take) mono as your tippet and that should break well before your rod. My guess is you’ll find that Smallies (and the Sunfish that live in streams with them) are a blast on that little rod.