Q: How to use a 4K monitor as a SECOND display w MBA?

My objective: the screen size on my 13"MBA is too small to handle larger Excel tables. I want to have an Excel table opened on the large external monitor, and my emails on the MBA, and move my mouse pointer between the two as I work with the files.

I purchased a 24" Dell p2415q 4K display recommended on apple.com to be used w my MBA (late 2015, El Capitan 10.11.6 at the moment).

I've connected the display with my MBA using provided Thunderbolt/DP cable. The display and computer recognised each other and so I went for Display/Arrangement in System Preferences. However, here I found I only have two options:

1) "Mirroring"  - the content of the display of MBA is simply mirrored on the Dell. This option is useless for the above mentioned purpose, because I can only view the same content on the Dell as I am having on MBA, not to mention that the resolution is also same like on the MBA, so the text only gets physically bigger on bigger Dell.

2) an unnamed option, when the upper menu bar can be placed EITHER on MBA display, OR on the Dell.

2a) When I left the menu bar on the MBA, then I only got the ElCapitan wallpaper and the monitor Preference pane on the Dell display - it seems closer to what I need, but unfortunately I cannot find a way how to get mouse pointer from MBA to the Dell display, so I cannot do anything with the content of the Dell display.

2b) When I placed the menu bar to the Dell display in the Preferences, then I could choose different resolution for the Dell display, so I could finally use the Dell display to its 4K advantage. However, the same resolution has also been applied to MBA and it made the text on MBA illegible. And again, as in 2a), also here there was no obvious way how to move the pointer from the Dell display to MBA display. Even worse, while being on the external display in the Display pane, I put the menu bar back to MBA, so the content went back to MBA, but the mouse pointer stayed in the Dell - I had to disconnect the Dell display in order to get the mouse pointer back to MBA display.

In windows, you just move the mouse pointer here and back between the monitors, easily. How to achieve this on Mac? Is there a shortcut for it or what do I need to do?

I thought the problem was that I had three Desktops (F3), so I deleted this and now I only have one Desktop - but the problem still remains.