I don't think Smash Bros. 4 will be quite as console moving as it's been in the past, but the 3DS won't be the reason for it. Smash Bros is probably the fighting game most closely associated with local multiplayer in the industry, people who buy Smash Bros are likely the same people who will be getting some friends to sit down and play it on the couch with them.
For the 3DS to have a serious effect, it would have to have some way to compensate for the lack of local multiplayer. Online multiplayer is one way that could happen, but this is Nintendo we're talking about here.
(And yes, I am completely discounting the possibility that anyone will get 3 or 4 people to all sit in the same room and play a match of Smash Bros, each on their own 3DS, because that stuff never happens)
