I'm confident that some form of Smash Bros. will be coming to the Switch. Whether it's a port of Sm4sh or Smash 5 remains to be seen. I love Sm4sh, so I would love to see it ported to the Switch and have it include all stages from both Wii U and 3DS versions, and hopefully DLC too since I didn't buy most of it on my Wii U (my GamePad is busted so no eShop transactions).
I think a port is more likely, though, because it would take a while to create a whole new game. We'll probably see it released either late this year or sometime in 2018. I think 2018 might be more likely because there are already a lot of heavy hitters this year, so releasing next year could help space out games and prevent droughts. I have plenty of games I plan to get this year for the Switch, so while it would be great to get a Sm4sh port this year, I'll be busy enough that I won't miss it if it doesn't come.