You can divide your mesh into several subtools and then use the Multi Map Exporter>Merge Maps option to merge the maps back together. The plugin will merge those maps that share a UV space so if your model already has other subtools make sure you do the maps for those separately.
Also, you may find that pressing Preferences>Mem>Compact Now before you try to create the maps helps.
ZBrush is a 32bit program so can only use 4GB of the memory on your system. And 35 million polys is a lot for it to cope with. It is also overkill for a 4K map. A 4K map won’t capture all the detail in 35 million polys because there are only 16 million pixels.