Unfortunately, these 4 to 6-hours delays are part and parcel of the TK longhaul experience. Every single night, a number of longhauls that are supposed to leave between 1am and 2am will be delayed until 5-8am because TK has no aircraft available and just waits for some of the overnight arrivals into IST. It seems the airline purposely arranges its schedule like this and to the best of my knowledge will not rebook on any other airline but will just let passengers wait at IST or at the outstation. Just for reference, as an indicator how systematic these delays are, info for the past week and only for the overnight departures from IST:

Fri, Dec 20
TK026 IST PVG delayed 5 hours
TK044 IST CPT delayed 5.5 hours
TK056 IST CGK delayed 5 hours
TK072 IST CAN delayed 6 hours
TK084 IST MNL delayed 5 hours
TK157 IST MIA delayed 6 hours


Sat, Dec 21
TK070 IST HKG delayed 5.5 hours
TK072 IST CAN delayed 5 hours
TK090 IST ICN delayed 4.5 hours
TK162 IST SGN delayed 5 hours
TK164 IST HAN delayed 6 hours
TK172 IST HKT delayed 4 hours


Sun, Dec 22
TK024 IST TPE delayed 5 hours
TK026 IST PVG delayed 4 hours
TK044 IST CPT delayed 5.5 hours
TK056 IST CGK delayed 4 hours
TK066 IST DPS delayed 4 hours
TK072 IST CAN delayed 5 hours
TK088 IST PEK delayed 6 hours
TK162 IST SGN delayed 5 hours
TK164 IST HAN delayed 5.5 hours
TK176 IST MRU delayed 4 hours


Mon, Dec 23
TK026 IST PVG delayed 7 hours
TK044 IST CPT delayed 7.5 hours
TK072 IST CAN delayed 6 hours
TK084 IST MNL delayed 6 hours
TK086 IST KIX delayed 3.5 hours
TK088 IST PEK delayed 6 hours
TK090 IST ICN delayed 5.5 hours
TK176 IST MRU delayed 3 hours
TK748 IST SEZ delayed 5.5 hours


Tue, Dec 24
TK026 IST PVG delayed 4 hours
TK044 IST CPT delayed 6.5 hours
TK054 IST SIN delayed 6.5 hours
TK066 IST DPS delayed 6 hours
TK090 IST ICN delayed 5.5 hours


Wed, Dec 25
TK088 IST PEK delayed 6 hours


Thu, Dec 26
TK024 IST TPE delayed 3 hours
TK044 IST CPT delayed 3.5 hours
TK072 IST CAN delayed 5 hours
TK090 IST ICN delayed 5 hours
TK172 IST HKT delayed 2.5 hours

I have long since started telling people to book a different airline. This has been going on for years and TK seems to have no interest in rectifying the issue. As a matter of fact, it seems to work quite well for them.