how are you raising it 4 inches off the hollowcore slabs??
have you poured a screed over the slabs??
insulation and draughs are not necessarily corelated.... airtightness comes down to standard of build, draughproofing and lack of services puncturing the building envelope.
i would only recommend insulating over the hollowcore slabs if you are trying to thermally break the first floor from the ground floor... i assume the first floor is private bedrooms.. will these be zoned seperately for space heating??
are you bringing the services below the slabs???... in my opinion this is the best practise...... i wouldnt recommend bringing them over the slabs..... if you are, be very careful as to not have services crisscross over each other, ie rad pipes and drainage pipes.....