@Nys_Ricardo
You can use Project to schedule your time/activities with each client and track progress and cost, if, you can adequately define client activities. I would think that type of work is "open-ended", meaning you won't always know what the "task" is so it becomes more of an allocated time (i.e. scheduled meeting) with the client. From that you may be able to identify specific tasks that can be quantified such that progress can be tracked. But, it all depends on what you hope/expect to get from Project.
As far as using subprojects, you only have 3 or 4 clients so unless each client's support evolves into multiple defined tasks (e.g. 100 tasks for each client), I suggest you keep everything in a single file with each client's activities under its own summary line (i.e. separate summary group for each client). Subprojects have overhead which you probably don't need. For example, they are a linked structure and prone to corruption if not managed with the utmost discipline. However, if you need to share plan details with each client, independent of the other clients, then using subprojects may make sense.
My thoughts, others may jump in with their suggestions.
John
