XPS 8900 PCIe Slot Questions

I have a Dell XPS 8900 purchased about a year ago.  I am exploring some possible upgrades for the XPS 8900 and have questions about the PCI Express slots on the system board.

The first slot (SLOT 1) is blue and appears to be an X16 slot.  It is where the supplied Nvidia graphics card was factory installed.  I assume that an upgraded graphics card (Nvidia GTX-1050) would go there as well.

There is another full size slot that appears to also be X16.  It is black in color and is labeled as SLOT 4 in the Dell service manual.  However, according to the Dell specifications for the XPS 8900, there is only one true X16 slot on the system board.  The second slot that appears to be X16 is in fact only X4 (SLOT 1).  Is that correct?

Finally, the smaller slot (SLOT 3) is identified as being an X1 slot, but I have read elsewhere on the forum that it is actually X4 (four lane).  I am planning to use an Addonics ADM2PX4 adapter for an M2 Samsung SSD drive.  The Addonics card requires a four-lane slot.

The purpose behind upgrades like this is, of course, improved speed and performance.  However, no chain can be faster than its slowest link, which is why I am asking these questions.

Thanks for any help you can offer.