I agree with Mr. Miller. You will not have to pay spousal support. Four years is not long enough to qualify under any scenario assumable from the facts you've given. Also, I doubt you have an informal ("common law") marriage. Those are not easy to establish.
BUT, if there's any question, go ahead and file for divorce. If the court finds there was no marriage, you are done. Or when the court grants your divorce, you are done.
What you don't want is to walk away, accrue a bunch of property, and later find that you were, in fact, married and all that you have accumulated is community property. There are some time frames and presumptions that work in your favor, but why gamble? File for divorce, no kids, no property, and then know that you are done with any further obligations.
First part of your questions regarding common-law or informal marriage:
In Texas there isn't a specific length of time for an informal marriage. Section 2.401(2) of the Texas Family Code states that the following can be used as proof of an informal marriage "the man and woman agreed to be married and after the agreement they lived together in this state as husband and wife and there represented to others that they were married."
So you have three elements: 1) the man and woman AGREED to be married, 2) man and woman LIVED together after the agreement, and 3) the man and woman REPRESENTED to others that they were married.
While the above are the basics, sometimes there are other exceptions or twists in the law that change whether a person is informally married.
Second part of your question regarding alimony:
Texas does not have "alimony" like many other states. We do have a concept called "spousal support" that is available in certain situations such as when the couple is married for 10 years, or if there are certain special needs of the other spouse. Because you said you've only been living together 4 years, you likely wouldn't have to pay "spousal support." But, if you have had children with your girlfriend, she can try to force you to pay child support even if you are not informally married.