Yes water is good on an empty stomach, I drink it myself occassionaly, with with honey/lemon. What I'm saying is that there is no need to drink 4+ cups of it. 1-2 is sufficient inshaAllah. When I said I didnt think it was wise to drink that much water, I had no idea why - until I just read the following...
Sitting down when drinking water
Drinking water standing up causes many ailments and doesn't quench the thirst, descends quickly to the stomach and does not settle so that the liver transfers it other parts of the body.
The Prophet (saw) recommended drinking water on an empty stomach. But he (saw) advised us to drink in three seperate breaths and to preferably drink it sitting down. (He (saw) drank water standing up once at the well of zam zam but there is no contradiction in the hadith and it is another matter to the one at hand.)
He (saw) used to sip water in three separate breaths and would say:
'This method quenches thirst better and is more platable and sanitary.'
This hadith indicates that the Prophet (saw) used to remove the cup away from his mouth, take a deep breath and drink some more. This method is better suited for the temperature of the stomach. (Imagine then, how unhealthy, drinking large amounts of cold water quickly can be?)
The Prophet's (saw) method of drinking water is better than drinking it in one breath (gulping), since gulping can dissipitate or weaken the stomach's natural heat.
There is also the danger of choking when water is gulped down. Cold water suddenly descending on the liver will weaken it, hence sipping is better. For example, if cold water is poured into a boiling pot of water it will decrease its temperature significantly.. but if it is added slowly the temperature of the boiling liquid will not reduce drastically.
There is more but I can't type it all out :exhausted
The prophet (saw) used to drink water with honey and this method is effective for preserving the health. It dissolves phlegm, purifies the stomach's protective layers and disspitates its viscidty and excrements while heating it mildly.
I think the Prophet's (saw) hadith provide the best guidence and this is what we should follow and leave everything else.
Caution: Drinking water with honey aggravates bile, so those suffering from this problem should avoid or add vinegar to neutralise the harmful effect of the honey.
There is an e-book available. I will find and post link inshaAllah. There is so much to be learnt from Prophetic medicine.
:wa:
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(Copied from 'The Prophetic Medicine - Imaam Ibn Qayyim al-Jauziah) in my own words here and there.)