SCHEDULE I Para 2 covers all possible situations of Para 4, then why it Para 4 is inserted ?

The difference is of" Import of Services" it's being clarified following in case of Import of Services in relation to related entities Under Para 4 : The Recipient who is Un-registered but engaged in business activities (say deals in Exempted supply)
is liable to do RCM on Import of Services & mandatory to obtain Registration as per section 24 of CGST Act (even though turnover is below threshold limit )

Note : Import of Services between Distinct Person is Exempted in term of Notification 15/2018 IGST (R)