Would it be possible to run 4 parallel capture cards directly into my iMac 2019 with Radeon Pro Vega 48 8 GB graphics and 3,6 GHz 8-Core Intel Core i9 and do a 4 camera live stream in HD30p?
Sure.
Get a BLACKMAGIC DECKLINK DUO 2 and a SONNET ECHO EXPRESS SE I
Works fine for me with a rMBP 15", 2013
Thank you so much!
And that’s a great idea with the PCIe to thunderbolt solution! 
And if you want to go crazy, get the DeckLink Quad which gives you 8 FullHD SDI inputs.
How much do you think my computer can handle? Do you think I could I run 4 4K cameras with the Dekclink 8K Pro?