Let out house property

First of all what is the meaning or conditions or rules for the word LET OUT.

CASE: I have  4 houses in my own name. One house is for my own residence. in case of second house not occupied by me for the purpose of residence, i visited this house 10 times during the year. In case of third house property there are 3 floors where in ground floor i put my goods related to my business and balance portion is fully empty. and in case of 4th house, house is totally empty during the year neither i visited there during the year.

My question is whether 3rd n 4th house is deemed to be let out. In case of 4th house can i say that this house is let out but vacant for the whole year as i didn't find any tenant and can save Tax under the head House Property and can remove the limitation of Section 24(b)

Please mention relevant sections case laws etc to support your answer