Just for the log: The 4200 mAh and 8400 mAh 4S LiFePO4-packs are back at Hobbyking. Now with yellow heatshrink and labelled “compact” despite the size is exactly the one of the older flightmax series:
https://hobbyking.com/en_us/zippy-compact-4200mah-4s2p-30c-lifepo4-pack.html
https://hobbyking.com/de_de/zippy-compact-8400mah-4s2p-30c-lifepo4-pack-w-xt90.html
73 de Michael, DB7MM