GEN. GUDU said:
Dear Friend
Dont worry about this now you will get medical I am sure (INSHA-ALLAH) about this, but it is matter of time. In my opinion you will get it within next two months from the date of your interview. I am telling all these as per my experience and observations. Now Your Selection is passed on the day when 2nd line added and your date of interview.
Now I come to your points about background checks Capt. Qorax has already explained it very deeply. After your Selection They go through some types of checking of your documents like Education, Civil, Job, etc,, (it depends on each VO and applicant some time just normal some time deeply or some time interview). And 2nd phase is security check which comes after medical received by VO.
In your case something was raised during your paper screening and thats way they called for interview. Now your 1st phase of background is almost over and if any it will not be too long.
About the 4 months as VO said, it is their standard time also written in their letter when they ask for any additional document so dont worry about it. Just be ready for medical within two months (INSHA-ALLAH).
Regards