I have upgraded to Devonthink 3 and I am using 2 desktops (development / office) and 2 notebooks (personal and professional) together with one iPad Pro. This zoo is because of some historical reasons.
Because of new license model, I need 2 additional seats for my pro license but this is too expensive for me actually. Because I need the Pro functionality only at my 2 desktops machines, I would like to buy two personal license for my notebooks. On my notebooks, sync of the files and all KI functions are the important features.
Note: To make sure we’re all talking the same language (and to avoid confusion for other readers), there are three editions for DEVONthink 3: Standard, Pro, and Server
If you buy a license of DEVONthink 3 - regardless of the edition - it comes with two seats, i.e., two per-machine licenses.
So you can buy DEVONthink 3 Standard and simply install it on the two laptops, as you have two seats with the purchase.
And there should be no compatibility issues with syncing the data between all four machines.