If you are trying to achieve between-subjects conditions, there are a couple of ways to do it.
1. As you suggest, you can make 4 separate files. These could be run individually, or you could embed them into different conditions of a Medialab experiment (which would be especially useful if you wanted to keep some aspects of the procedure constant, such as initial instructions or an individual difference measure at the end of the study).
2. Alternatively, you can have one DirectRT file. To do this, you can include an initial item asking for assignment to a condition (to be completed by the experimenter before the session). This should then be followed by all the trials for condition 1, those for condition 2, and so on. In the initial assignment item, use the skip function to move the participant to the first trial of the appropriate condition. At the end of each condition, include an unconditional skip to move participants over the remaining conditions. I have included a small example to show you what I mean. Each condition is just three trials (instructions, response, and a thank-you), but it could easily scale up. Also, note that the condition number will be the first response in the data file for each participant.
Do these work with the design you had in mind?