I just built 2 projects to your specs. Started with an empty project, and all I added were either 4 Instrument Tracks with a Kontakt instance on each; or 4 midi tracks all pointing at a single rack instance of Kontakt. Each had one violin, viola, cello & bass.
I then closed Cubase and launched it fresh for each project by double-clicking on the project while using Task Manager (PC) to see the memory used.
Midi tracks used 665 Mb
Instrument tracks used 797 Mb
That works out to be about 33Mb difference per track. Assuming these numbers are representative and they scale linearly (unlikely), for every 30 MIDI Tracks you change to Instrument Tracks you’ll use 1Gb more memory.
For my typical track count that is more than a fair trade-off for the convenience of using Instrument Tracks. For folks using high track counts the opposite would be true.