It is unusual for a
TN to be granted to an employee of a Cdn company.Typically a B1 is sufficient. I take it you are working for a US client in the US.
In any event, you would only be subject to US tax if you spend more than 183 days in US in any given 365-day period. So you need to literally count the days you spend in US, from the time your firm started working on this project. If you want to avoid having to file or report income in US, you need to make sure you are in canada 183 days out of ANY 365-day period. If you spend 4 days a week in US, you will run up against that 183 day limit. This is a recent change in US/Canada tax rules. Before, as long as you were being paid by Cdn firm and did not have a permanent establishemnt in US, you were US tax exempt regardless of the number of days you were in US.
Note that you may have to pay state income tax regardless of how few days you spent. This would be a matter for the state to decide.
forums.serbinski.com is a place better suited to this type of question.