... but I am in a job that doesn’t allow work outside of US.
You say your job doesn't allow work outside of US but you havent' quite clarified why - in this scenario - you woulnd't just live in Canada and commute instead of specifically residing in Port Roberts so as to be able to commute while crossing into Canada frequently (yes, gaming the system). But anyway, that's up to you - your reasons are your reasons.
As
@dpenabill notes, this is probably more difficult in practice than in theory. In theory, yep, any part-days in Canada count as a full day in Canada. In practice it's just impossible to say how much difficulty you would have crossing the borders
four times a day. Despite what some might say, border officers (in both countries) can basically stop and question you for as long as they'd like, inspect your stuff, etc. (Yes, there are some limits - they won't hold you for a week - but since we're talking about four interactions a working day - even extended questioning 1 in every 100 crossings is just a bit less than once a month.)
I'd also note: even if this might work for years, things could come up that would make it completely unworkable - notably covid meant some very, very inconvenient and problematic situations for Port Roberts residents. Just as an example.
Which is all fine if those are risks you're willing to take and deal with it as these things come up.