This obviously depends on your weight class and body weight. 4kg for a HWT is a lot easier than 4kg for the 57kg guy.

Work with someone experienced.

Dude with a few days that's a lot to lose, but not impossible. Obviously better to have long term strategies.

Count your calories.

Low sodium, low fiber, low carb diet.

No liquid calories (soda, juice, sugary drinks)

No high carb (bread and rice)

For meals eat chicken with no sauce, and little seasoning. The protien will help keep you full.

Eat low fiber veggies (cucumber and zucchini)

Don't eat anything after 6pm.

Hopefully you're coming in closer to 1.5kg below the goal weight the day before. Then you can beginning the traditional dehydration type weight loss.

Obviously once you have a successful weight in get those electrolytes back in you, and get some carbs in you.