If all self employment income is claimed on the T4, why do i need to enter business details that would be part of business financial statements

If you are self employed, then the only business statement that you will be submitting is the T2125 on your personal tax return. 

If your business is incorporated, then your T4 shouldn't have self-employment income on it. You would need to amend the T4 that you gave yourself so that the income is in the regular employment boxes, not the self-employment boxes.

If  3rd party gave you a T4 slip, then it's self-employment income, as the T4 is for individuals, not incorporated businesses.