in my iPhone 4 the screen is very dark

Sounds like the backlight has gone! The backlight to your iPhone is probably the most energy-intensive component to your iPhone. It requires alot of power and will have a minimum operating voltage of only a few volts. I see that you have two options;

1) Place your iphone with the back and preferably the logic board (as much as you can really) removed and place it into a dry bag, placing it in a bag of dehydrated rice and then in a dry, warm room like the drying room or pantry overnight should remove any ecxess water or water vapour.

This would be a lucky fix but, only cost you 99p.

Your second option; buy a new display assembly and replace the LCD with its back light completely. This has a much higher chance of working and although very fiddly, it will most definitely (99% sure) work.

Just make sure you order the right one! (either the 4G or 4S) as the two are not compatible unless you want to use an angle grinder and force-fit it, which I as a novice have done and succeeded, but I wouldn't recommend it!

Hope this helps!