I have Lg monitor which i can split into 4 displays. I am using dell Tb 16 240w docking station. When i try to connect to 4 monitors, At a time I Can see only 2 display. Does mst hub solve my issue?
The XPS 15 9560 will support an MST hub if it's plugged directly into the system's USB-C/TB3 port. The TB16 dock does not support MST on its DisplayPort outputs. However, using a DisplayPort MST hub on the system's USB-C/TB3 port will only allow you to tap into the display bandwidth of a single DisplayPort 1.2 channel. When using Thunderbolt 3 to a TB16 (or to a Thunderbolt 3 to Dual DisplayPort adapter), you get the bandwidth of two DisplayPort 1.2 channels -- so an MST hub would probably not be a better solution here.
When you say you can only see two displays on the LG display, are you also keeping the laptop's built-in panel active? If so, the limitation here is Intel GPUs currently only support a maximum of three independent displays simultaneously, regardless of resolution or how what display output they're connected to. Even if your XPS 15 has the NVIDIA GPU, it's not actually wired to any of the display outputs because it operates as a render-only device using NVIDIA Optimus, so it doesn't come into play here. If you disabled the built-in panel, you would be able to get a third "panel" of the LG display active, but there's no way to get 4 independent displays running from that system with all of them driven the Intel GPU. The only way to get additional displays beyond what the Intel GPU supports would be to use something like a DisplayLink adapter, i.e. a DisplayPort/HDMI to USB-A adapter, which you could plug into the dock's USB ports. This works because displays attached via DisplayLink technology are not directly driven by a GPU, but it also has some significant drawbacks that I wrote up in this thread, specifically the post marked as the answer.
If i plug MST to Thunderbolt port can I also use HDMI port available?
@nikunj3685, yes you can, although keep in mind that the Intel GPU only supports up to 3 simultaneous independent displays, regardless of how they're connected, and all display outputs on that system are wired to the Intel GPU, so even if you have an XPS 15 version with an NVIDIA GPU, that won't change this limitation. So for example if you want to have two displays connected with an MST hub and a third connected via HDMI, you would have to disable the built-in panel to run that combination.