How difficult is it to switch majors going from a BA to MA/Phd? A few years ago, I received a BA in English. While I was studying my focus on English, however, shifted when I took a class on literary theory in the latter half of my 3rd year, which inspired in me a deep passion for critical theory. I continued to read and study in this area, often filling my electives up with classes that related to it, but I never switched majors over to something more akin to my new found interests like philosophy or sociology because I felt it was too late to switch majors. 

At this point, I would like to pursue an MA/Phd on this subject in one of the aforementioned majors, but I am concerned that it may be difficult for me to enter into a program with a degree from a different field of study. I'm also worried about the fact that I've been out of school for about 4 years now because I've been told that being out of school for an extended period of time can be a real obstacle to getting into a program. Any help or advice would be greatly appreciated.
In the US there is normally little problem in chaining fields after a BA/BS degree. It is very common. This is partly due to the general nature of the baccalaureate in the US. For example, a math or science major also has quite a few courses in the other as well as in the social sciences and humanities. And someone majoring in the humanities has an even broader view of that and the social sciences. 

However, note that you may be missing a few things when you begin graduate study. This is normally covered by the advanced courses that you take starting out in either a masters or doctoral program. I think a change such as you desire wouldn't be a block at all. 

But you will need to measure up on all of the other things such as GPA and letters of recommendation. Doing well in related courses will probably be considered. 

But the only way to know for sure is to apply to a few programs and see what feedback you get. 

Having been out of school for a while isn't necessarily a block, but if you have gotten rusty in your field, then you will have a harder time getting back into the groove. And letters may be harder to obtain, so try to reestablish contacts with professors before you apply. They can also give you advice. 

Your statement of purpose in returning to study will probably be important.