4 days late on my period negatice tests and its been 3 weeks since i had protected sex should i be concerned??

If pregnancy occurred as a result of sex three weeks prior to the expected onset of your period it is a minimal risk for pregnancy. Ovulation may be delayed, also delaying the period, but that would have occurred even later in this cycle, again making the risk of pregnancy minimal to none.

Without knowing about the length and regularity of your mentrual cycles and without information on your contraceptive methods/means of "having protected sex", it is not possible to answer your concerns. If you want to find out whether you are or are not pregnant, you may seek a blood betaHCG pregnancy test, which is among the most reliable ways to detect an early pregnancy.