一年に、四回は旅行します。 ↑This is the correct sentence. At least four times, I will travel between 1 year. ↑Meaning is similar.

I think that I am going to explain the same thing with your teacher though.... Particle 'は' indicates the function of the preceding word '四回'.It indicates the topic. Therefore, th word preceding は often becomes a subject. は is translated into English as "as for..., talking about..." etc. eg) Watashi wa John desu. = Talking about me I am John. Seeko-san wa Nihon jin desu. = As for Seeko she is Japanese. Hence, you might be able to understand は which is in the sentence you pasted as follows. Taking about four times it is frequency of my trip in a year. or As for four times it is frequency of my trip in a year. Does it make a sense for you??...