I can not add the 4th monitor to my system

Nice to meet you, *****!
What troubleshooting have you done so far?

Are you using Windows 10, right?

Got it.
The first thing you need to do is connect the monitor to your HDMI, DVI, or VGA port on the PC.
Have you already done it?

I see.
Have you tried to connect your Sabrent adapter to HDMI port on the PC?

Please, try it. Seem like your Sabrent adapter suppose to be connected to an HDMI port, not USB.

Got it.
Have you already tried connecting your 4th monitor to the adapter?

Then press Windows Key + P on your keyboard. This will bring up a menu with a list of options.
In the example above the second monitor is on the right.

Have you tried to follow the instructions above?

And your monitor still doesn't show up in the list?

Most likely, you need to update the driver software of your monitor. Please, connect your 4th monitor, then choose it in the list. In the search box on the taskbar, enter device manager, then select Device Manager. Select a category to see the names of devices, then right-click the one you'd like to update. Select Search automatically for updated driver software. Select Update Driver.

Is there anything else I can do for you today?