Thanks for all your responses and advice !

I ended up finding a dark corner with a bench at T4 and slept comfy for 3 hours until check-in lines for my flight started at 3.30 am - could not find the mentioned 'quiet rooms'. There was also heavy drilling and construction noise throughout the night.
I saw the Yotel but decided not to - was too much for my budget at £59.- for 4 hours.

Although being technically a female 'senior citizen', I am glad I opted for the bench - piece of cake - and noticed that many others followed suit ! ^