A sizable portion of my working shift is dealing with blocked catheters, mainly because people haven't drunk enough.
We get a noticeable number of call outs in hot weather.
I would agree 4 litres a day, especially in the current hot weather.
I would count any fluids in that; water, squash, tea, coffee. Although Tea, coffee & cola have a diuretic effect there is still a net gain in fluid intake so they're all good.
I'd also be happy for beer/cider to be included provided it's not otherwise contra-indicated and is in moderation. Again it's all fluid intake and keeping you hydrated.
It is possible to overhydrate and get electrolyte imbalance. It can be equally as dangerous as dehydration, so just something to be aware of, especially if you're doing endurance cycling or other activities in hot weather.