Again, the health insurance premiums are not taxable on the federal level, so you would end up paying tax on something that shouldn't be taxed.
Also, if you change your T4, the CRA would likely just change it back, since it doesn't match the T4 that they have on file.
If you think that there is something incorrect on your T4 slip, then you can contact your employer to verify if that is the case. Then they can get you a corrected slip if needed.
