The TREK1000 default configuration as supplied from Decawave supports 8 tags and up to 4 anchors. If support of additional tags is necessary, you should refer to the document “TREK1000 Expansion Options Instructions” available on http://www.decawave.com/support
One can also develop their own tags using our DW1000 IC or DWM1000 module. To assist customers with PCB layout of a DW1000 based product, three files in DXF format are available from Decawave. See APH006 PCB Layout available on http://www.decawave.com/support.
Yes, it is possible to use a 4th anchor.
However, there is only one use case for the 4th anchor, and that is when one wants to locate the tag above the plane of the anchors. If the tag is always below the plane of anchors then the 4th anchor is not supposed to be used. It will not result in any improvement in the location estimation.
For further details see the section 8.2.1 adding a 4th anchor in the TREK1000 user manual.