"četl dopisy od dvanácti do čtyř" je zde předložka "ty" doopravdy z pohledu češtiny nezbytná?
ano, jinak je to jen "He was reading letters"
Is it possible to say "Četl ty dopisy od dvanácti do čtyřech hodin"?
Yes. It is a bit colloquial.
Thank you again for your help!
Is it possible to use 24-hour time ("do šestnácti") in a sentence like this?
It is. It is less common in informal communication.