Hi!
You can select a quality level from the dropdown in the Fab UE plugin to get the resolution that you need. For instance selecting Raw quality will give you 8k resolution. More details here: Updates: New in Megascans
About only seeing over 14k assets in Bridge plugin for UE as compared to 18k in the Megascans library on web, it is because a few asset categories (Atlases, Displacements and Brushes) are not supported inside the Quixel Bridge plugin.