Quadruple

QUADRUPLE LETTERS
The first citation for "kiwi" in the OED^* is from 1779: "The birds of these islands are as beautiful as any we have seen... Another is of an exceeding bright scarlet colour;..its native name is eeeeve.

ESSSSE (an obsolete form of ashes) is in the OED2.

LLANFAIRPWLLGWYNGYLLGOGERYCHWYRNDROBWLLLLANTYSILIOGOGOGOCH, a town in North Wales, has been described here as having 4 consecutive L's. However, Alex Willcox writes: "I've just been looking through the article on multiple letters, and as a Welshman, I'm afraid I must take exception to the assertion that the name 'Llanfairpwllgwyngyllgogerychwyndrobwyllllantysiliogogogoch' contains four consecutive 'L's. In the Welsh alphabet, 'Ll' is one letter, completely separate, and pronounced differently to 'L'. (Other examples are 'Ch', 'Dd', 'Ff', 'Ng' and 'Th'.) As such, there are not four consecutive 'L's, but rather a double 'Ll'."
Site at llanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogoch.co.uk

JÄÄÄÄRNE is an Estonian word meaning "the edge of the ice."
KUUUURIJA is an Estonian word meaning "moon explorer." However, the words are usually hyphenated as jää-äärne and kuu-uurija.

BRRRR (an interjection expressive of shivering) is in the OED2.

According to Scott Oglesby, Japanese has a word for princess that can be romanized JOOOO, which has four "o" sounds in a row.

ZEE-EEND is the newer spelling of a Dutch word meaning "sea duck." Under the older spelling it was ZEEËEND.

^* OED = Oxford English Dictionary