The Mega ST 4 "Low" uses an MCSWITCH adapter (ATARI Monochrome & Color VGA Adapter) by Olivier Gossuin. It shows all three ST resolutions on a suitable VGA monitor, with a switch for mono and color. The version documented on albersdoerfer.de also has an A/V connector and volume buttons, but mine is an older revision without the volume buttons and with a slightly different look. I can't find my exact version at Olivier's anymore, nor a newer one, so the adapter I have is a bit of a rare older revision.
The Mega ST 4 "High" uses a self-built adapter. I followed the guide at pest.atari.org to build a simple RGB-to-VGA adapter for the Atari monitor port.
+The Mega ST 4 "High" uses a self-built adapter. I had to build my own because every off-the-shelf adapter blocks the VGA slot of the ET4000 card, since both connectors sit at the same spot on the machine. My solution is a short cable leading to a small box, which carries the VGA connector and the mono/color switch, so the ET4000's own VGA output stays accessible. I followed the guide at pest.atari.org to build the RGB-to-VGA adapter itself.
I also bought a new active ST2VGA Enhanced from SidecarTridge, but it isn't in use yet. Unlike the passive adapters, the Enhanced adds an active amplifier and filter stage for early or noisy Atari machines, and carries its own +5V supply over micro-USB. It's designed for machines with weak video output, visible "jailbars", or picky LCD inputs that fail to lock to the signal.