USB-C Docking Station - Dual Display Max Resolution

@GoChanGo  Yes, there are bandwidth limits.  And sometimes there are limits in terms of the outputs that can be used together because the dock can only allocate display bandwidth in certain combinations.

As to any limits of your particular setup, I can't say because unfortunately you didn't specify your dock model, which is a key piece of information in this case.  But if you go support.dell.com, search your dock model, click Documentation, and select the User Guide, you will find this information spelled out there.  (As a general piece of advice, Dell's documentation is generally quite good, so it's a good resource to check when something isn't behaving as expected.  Sometimes the answer is already available rather without having to ask the Internet and hope you receive a correct answer.)

But basically, the Latitude 7400 is a DP 1.2/HBR2 system, which means that:

• With a regular USB-C dock like the standard WD19, dual displays would be limited to 1920x1080 60 Hz each.  If you have a 4K display connected, that display might be running only at 30 Hz rather than the standard 60 Hz, which you might notice as seemingly laggy performance.
• With a Thunderbolt dock like the WD19TB, you can run dual displays up to 4K 60 Hz each if you connect them to an allowed combination of outputs.  In your system's case, one of them would have to be connected to the downstream TB3 port, using a USB-C to DP cable/adapter or similar if needed.

If you want to know more about bandwidth limits and why they exist, I wrote an explainer post on the modes of USB-C and TB3 with specific focus on their impact on display setups over here.

Lastly, a 4K display and a 1080p display are generally not going to be a good match.  Unless they are vastly different sizes, you're almost certainly going to need to set different scaling factors on each display to keep things a reasonable size on both, but running multiple scale factors simultaneously results in non-optimal image quality on any display that isn't running the scale factor of the display that was primary when your user session started.  Basically those other displays won't look as good as they would if you were running those by themselves.  This has to do with how Windows rendering works.

Thank you for the quick reply @jphughan !! The docking station I'm using is VAVA 12-in-1 Dual 4K Docking Station. The reason why I have a 4K (primary monitor) and a 1080p display, is that the 1080p I use it when I need to screen share during meetings, otherwise the viewers have difficulty seeing a 4K screen.

@GoChanGo  Happy to help.  In terms of your docking station, VAVA's product page doesn't provide clear technical info, but based on the specs that note that only 2019 MacBook Pros can run 4K 60 Hz while older systems will drop down to 4K 30 Hz, and that even with newer systems the possible "dual 4K" setups relate to either "4K 60 Hz + 4K 30 Hz" or "mirrored 4K 60 Hz" (as opposed to dual independent 4K 60 Hz), it appears that the docking station is a regular USB-C, non-Thunderbolt dock that supports DisplayPort 1.4 over USB-C.  But your system only supports DisplayPort 1.2 over USB-C, which is why you can't use the full capabilities of the dock.  However, since the Latitude 7400 supports Thunderbolt 3, you have the option of using a TB3 dock instead, which again can run dual 4K 60 Hz displays even from a DP 1.2 system.  So you might want to go that route.  If for some reason you don't want to look at Dell docks, CalDigit has the TS3+ and the USB-C Pro Dock.  Both of them support TB3.  The former REQUIRES the system to have TB3, while the latter will fall back to regular USB-C if needed, although it's strange that they gave it a name that doesn't indicate TB3 support at all.  The available ports on the two are different though.  With the TS3+, you'd need something like a USB-C to DisplayPort cable if you wanted to run dual displays.

Note however that one perk of a Dell dock is that you can use the Power button on the dock to control the system.  Some people like being able to power on or wake up their system while the lid remains closed.