Looks like @splillo is correct - Delta is the fastest storm to intensity from a tropical depression to category 4 hurricane in modern Atlantic records. The previous record was Keith 2000 (42 hours).

We may break the Category 5 record as well (54 hours, tied Felix 07 & Wilma 05). pic.twitter.com/SaM6Hc60Rv

— Tomer Burg (@burgwx) October 6, 2020