I thought Amazon wasn't going to update any Kindle in 2018 but I got it wrong. Amazon, with little fuss, updated the Kindle Paperwhite. The Kindle Paperwhite 4 is a typical Kindle update, i.e. incremental with just enough to justify an overdue refresh.
The Kindle Paperwhite keeps the six-inch display (something expected considering the 'premium' Kindle Oasis 2 has a seven-inch display). Storage goes up to 8 GB or 32 GB for the extra space required due to Audible support. There is also water-proofing, Bluetooth support, an extra LED for the front light, an adjustable front light and the weight is decreased to 182 grams.
The Paperwhite 4 is also more robust than the previous generation and is designed to withstand small drops. There is no indication that the Paperwhite 4 is using E-Ink Mobius, so it is not clear what Amazon did to make the screen more robust. Due to the extra features and storage, Amazon has increased the pricing from £109.99 to £119.99.