Actually they are all perfectly correct. Sentences 1 and 4 are split infinitives.
Most grammar experts ignore the rule against split infinitives. It is a fake rule.
It comes from the fact that in Latin it is impossible to split an infinitive. There is no reason why English speaker should follow this rule. Often splitting an infinitive is quite useful for emphasis. The rule was popularized by a Grammar book called "Strunk and White". Most grammar experts think this book is wrong in several more areas.
American speakers split infinitives quite often. The phrase "to boldly go where no man has gone before" sounds much better than any of the alternatives.
However, if you have a teacher that says that you shouldn't split infinitives, then anytime you see an adverb between the word "to" and the "verb", move it.
But as soon as you leave this class, forget this bogus advice.