reverse gst applicability

I am composition dealer. my query is on RCM. I purchased goods worth 4500 in a single day and worth 4900 on another day of same month. I sold the items.
now while filing gstr4 , I will show my unregistered purchase in given columns of the return. but the question is whether RCM will apply or it will be shown as zero? guide on this. the items were covered under 18% rate.