4K Support: 4K displays are currently only supported on a handful of MacBooks. In almost all cases you will have to step down the resolution or refresh rate in order for this to work. Please refer to the Apple support article here for a brief overview of if and how you could use a 4K display with your MacBook.
Horizontal Docks: There are one Thunderbolt 2 port, one mini DisplayPort and one HDMI port. If you'd like to connect two Thunderbolt displays, you will need to connect one display to the Thunderbolt 2 port on the dock and daisy-chain the second display off of the first display. If you'd like to connect to non-Apple displays, you'll simply need mini DisplayPort adapters to connect to your monitor (HDMI, DVI, etc) or use the HDMI port.
Vertical Docks for the MacBook/MacBook Pro: If you'd like to use a non-Apple display, you'll simply need mini DisplayPort adapters to connect to your monitor (HDMI, DVI, etc).
Vertical Docks for the MacBook Air: The MacBook Air is not capable of outputting to more than one external display natively but you should be able to add a second display by using a USB Graphics adapter. You can pick those up from most online electronics retailers and they are relatively inexpensive. Make certain that the one you select indicates it is compatible with macOS.
Please note: The dock for the MacBook Air is not compatible with Thunderbolt displays or peripherals.
Please note: The Vertical Docking Stations for MacBooks released prior to 2016, excluding the MacBook Air dock, are designed to fit Henge Docks and Apple-branded cables. We recommend purchasing one of these to ensure that they will fit in your dock. As the Horizontal Dock does not require cable installation within the dock, all cables should be compatible with the dock.
For the Vertical Docking Station for MacBook Pro with Retina Display (2012-2015), if you wish to connect to a Thunderbolt monitor, you'll need to remove the cables that came preinstalled in the dock, as these are Mini DisplayPort cables. It's a very simple process that's detailed in the User Guide.