Here is a list of current distributors for Pixhawk 4 (as of July 2018):

Banggood – Ships worldwide
Seeed Studio – US/GB/DE/AU Post, EU Post, Singapore Post; DHL; Fedex from Hong Kong
GetFPV – US shipping from Florida (USPS, Fedex, UPS); International Shipping (DHL)
Amazon (via DevTu) – US shipping from North Virginia
GroupGets – US and global shipping from Nevada
Unmanned Tech Shop – Same day dispatch in UK, fast delivery to most of EU
3DXR – UK, EU, and global shipping from North Yorkshire
Mybotshop – Ships worldwide via DHL or Deutsche Post
Synosystems – Ships worldwide via DHL or Deutsche Post
BuildFPV – Indonesia
DroneWe – Korea
Taobao (via GroundVendingStation) – China
SDModel – Taiwan