Used one power adapter for 4 devices
If all your 4 devices require 12V power source, and total combined current consumption will not exceed capability of your adapter (3A), then you can use this power adapter to power all 4 devices at the same time.  

There will be always some voltage drop in any cable. It just depends on what voltage drop you (or your devices) will accept.  

If you want to daisy chain power cable, of course the device which is most distant from power source will have the highest voltage drop. But it all depends on thickness of cables. The voltage drop could be negligible.