#1 is correct. #2 is correct. Abbrivated titles require a period after them, e.g. Mr., Mrs., Ms., Dr., Esq. #3 She dreaded April, tax season. Colons should not be used after a sentence fragment, e.g. "my favorite things: ice cream and cake. #4 should be "Three company computers are currently unaccounted for and will be reported stolen.

You need to consult a style manual. For example, adding a full stop after Mr is acceptable in some places, and unnecessary in others. I'd say only the third sentence uses standard and correct punctuation. The first one is only acceptable depending on the writer's style, and it certainly isn't standard.