Evidence for this being a brand new game:- Development of Sm4sh took 2 years. It's been over 2 years since the release of the last DLC, and if we assume that production began just after the final DLC (hell it could've started production while they were still making Sm4sh DLC) it's not unreasonable to believe they've made an entirely new game within these 2 years
- The logo is different and a placeholder. Seems like a lot of effort to just finish with "lol it's a port"
- HAL Laboratory, who didn't help with production of Sm4sh is credited - Namco Bandai, who helped make Sm4sh, is not
- The main website is essentially empty, with a brand new logo and Inklings trailer - there's no more SSB4 information
- Link is in his BOTW costume - why would they change his model/costume for a port?
Evidence for this being a port:
I honestly think there's none but I'm shocked it's likely to be a brand new game given how well definitive edition of Sm4sh + additional characters would print money and would be a much easier job