We recently spent a week at the Hilton Terminal 4 and on the second last night decided to splurge and give the Zen a try. We noted that the restaurant seemed quite busy each evening, the selection looked good and the prices (excluding the wine) weren't too outrageous, especially for an airport hotel, which has a captive audience.
On our chosen evening, I did a quick scan of Tripadvisor to see what others' had to say and noticed that the restaurant would to takeaway. So we went downstairs, verified that this was possible, placed our order and quickly went to the Marks & Spencer in T3 to get a bottle of wine. When we showed up to collect our order, it was just coming out of the kitchen.
The two dishes that we ordered (can't remember the exact name but one was a chilli beef and the other a spicy chicken) were delicious. The meat was tender and flavourful and one order of rice was more than enough for two people. I would definitely recommend this restaurant and if you do the cheaper option (i.e. takeaway) the prices are very reasonable.
I couldn't tell, but I am guessing that this is likely a favourite of those living locally and business isn't only relying on hotel guests.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.