http://hidot.hawaii.gov/highways/files/2014/09/Regional-Federal-Aid-Highways-2035-Transportation-Plan-for-the-District-of-Maui_Yong1.pdf
From page 20 to 22 are maps of Maui, Molokai and Lanai with several 3- and 4-digit routes I haven't seen before. For example, on Lanai not only 440 (and 44), but 430, 435, 441 and 442. Nine 4-digit routes in Lahaina, not counting the bypass.
Page 80: Long-range proposals, including adding 2 travel lanes to portions of HI 30, 31, 32, 37, 3100, 3180; and building a future 3960.
Page 101: Maui detail maps, including future HI 374 (Kihei to Makawao). More maps, page 102-108
Page 109: functional classification of highways; pretty much a high-level highway log by number, milepost, classification
Well, that was good... are there any for the other islands? Or statewide? Yes!
http://hidot.hawaii.gov/highways/other/other-related-links/hawaii-statewide-long-range-transportation-plan/
There are dozens of 4-digit highways in Honolulu. More 4-digit routes in Kauai. And the highest numbered highway in Hawaii (though not signed) appears to be HI 9285, Pualalea Street, from Kamehameha Highway (Route 83) to end of road. 0.31 miles.
Here's part of what you'll find:
(https://www.aaroads.com/forum/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FplAzHVb.png&hash=f71169d188767f7e5e873d88c4e51a370b7e7a2a)
Mele Kalikimaka, everyone!
Thanks! Just a quick note on many of the 4-digit routes you spotted -- many of them are county roads, with no signed route numbers, and the route numbers seem to be assigned by the state for planning and GIS purposes.
Some of the route numbers differ from what's signed in the field (or at least was, last I was out there), such as 3700 for what's signed as county 31 in southeast Maui, and 475 for what's signed (on small number plates in milemarker assemblies) as county 490 in north central Molokai.
So much for thinking I was close to clinching the network.