Which of the following is a soft metal that ignites in air and reacts violently with water and has atomic number 37 in the periodic table?

Rubidium

The correct answer is option 4. Rubidium.

The soft metal that ignites in air, reacts violently with water, and has atomic number 37 in the periodic table is Rubidium. 

Rubidium is another highly reactive alkali metal, and while it doesn't ignite in air as readily as caesium, it still reacts vigorously with water, producing rubidium hydroxide and hydrogen gas. Like other alkali metals, rubidium has a low ionization energy and readily loses its outermost electron to form a positive ion. Rubidium has atomic number 37, making it the correct choice according to the options provided.

Rubidium shares many chemical properties with other alkali metals, such as lithium, sodium, and potassium. It's a soft, silvery-white metallic element that is highly reactive and is typically stored under oil to prevent its reaction with moisture and oxygen in the air.