I fly ith KLm every 2 weeiks & always access T4 Holideck lounge.
Its the most fresh & spacipous (as on 3 floors) in the whol of LHR. great views with plenty window viewing areas, trendy seating, matched to moods for business, snoozing area, pc access, quiet lounge etc.
No substancial foods in there but all the basics healthy snacks & a great US style bar area on the 1st floor.
head up to the 1st floor where its quieter.
The observation declk has a panormaic view over the whole airports & the southern runways, nice to be quiet or watch the world a little, harfdly anyone up there.
Skyteam are developing the new lounge at T4, much works going on all throught the terminal, so interesting to see what comes soon.
But for me, its a unique design, much along the theme of KLM’s fresh modern art, functional, but warm colours & atmostphere.
Lucky form me i get Gold card access with any another traveller too, but certainly i would pay to go in there for a few hrs most certainly.
With the views & moods you can find to match your mode, its the best in LHR.
I hope you enjoy.