I am from Vietnam and intending to fly to Manchester (UK) with Turkish Airways with a layover under 4 hours at the Istanbul Airport (Turkey). Will I require a transit visa for the layover in Istanbul airport if the transfer time is under 4 hours at the airport?
No, as long as you don't leave the air-side area you don't need a visa. I spent a good 12 hours stuck air-side at Ataturk airport back in 2012 transiting on a flight back from Tokyo to London.
It's also worth noting that quite a few countries can get visa on arrival/E - visa for Turkey if you would like to leave the air-side area