In the following compounds. The decreasing order of acidity is

Acidity is more, when the phenol has more number of electron withdrawing groups or the groups that show -I effect. 

Nitro group is a strong electron withdrawing group and its effect is more when it is at para position and less when it is at meta position. Therefore, 4 is more acidic than 3. Methyl is an electron releasing group, so acidity of 2 is the least. Therefore, the correct order is:
4>3>1>2