Hi,
I think the aircraft category is defined by wingspan...
a cat G aircraft must have a wingspan above 80 m (An 225)
cat F between 65m and 80m (A380 / An 124 / C5 B Galaxy...)
cat E between 52m and 65m (777-300 / A340-300)
cat D 36m and 52m (A310-300 / B707-300)
cat C 24m and 36m... (B737-800 / A321)
A 747-600X or 500X or 400X QLR has a wingspan above 65 m, so cat F.
a 747-400 ER or 300 or 200 has a wingspan less than 65m, consequently it's a cat E aircraft.