Here's a suggestion ?:

To measure exactly 4 liters of water using a 5-liter bottle and a 3-liter bottle, you can follow these steps:

1. Fill the 5-liter bottle completely (it now has 5 liters).
2. Pour water from the 5-liter bottle into the 3-liter bottle until the 3-liter bottle is full.
   • Now the 5-liter bottle has 2 liters left, and the 3-liter bottle has 3 liters.
3. Empty the 3-liter bottle.
4. Pour the remaining 2 liters from the 5-liter bottle into the 3-liter bottle.
   • Now the 3-liter bottle has 2 liters, and the 5-liter bottle is empty.
5. Fill the 5-liter bottle again completely (it now has 5 liters).
6. Pour water from the 5-liter bottle into the 3-liter bottle until the 3-liter bottle is full.
   • The 3-liter bottle already has 2 liters, so it can only take 1 more liter from the 5-liter bottle

After this, the 5-liter bottle will have exactly 4 liters left.

Answer: 4 liters are now in the 5-liter bottle.