As you have mentioned you got the IUD inserted on 28th last month which i presume was after you finished your last menses.
Since its been 4days overdue for you in this cycle, i would suggest you should take up a home pregnancy
test of urine with kits available like velocit, i-can etc using morning sample of urine.
Also i would want to know if you can feel the iud thread inside your vagina. If you could previously feel it after insertion and now you cant , it indicates the iud must have been displaced or must have come out. Sometimes it might be that you get pregnant if the iud was not properly inserted.
I suggest you should do your urine test. If its negative, still you visit your gynecologist so that she does an internal examination for you to examine the thread of the iud as sometimes it just gets coiled up in vagina. Get an ultrasound
if thread is not seen even on examination.
Incase everything is normal, you just can wait for menses as sometimes menses do get delayed due to several reasons like stress
, change of place and environment.