I applied to 10 schools and have heard formally from 3, 2 rejections, 1 admission. I just received an email from another school (a very good one) saying that they are seriously interested in me, but would like to know if I am still interested in them. While I am very interested in this program, which is in my top 4, since I have not heard from the other 6 schools, I am not ready to make a decision. Does saying yes now mean I am committed to accepting their offer?
The department in question most likely just wants to make sure you haven't accepted an offer somewhere else - or received an offer you will definitely accept over them; saying that you are still interested is not a commitment from you at all.
Some applicants will wait till the last moment to decline offers from departments, even ones that they know for sure they will not accept (e.g. if they have been accepted by a better department (in their estimation) already). The department is just trying to find out whether you are in that position. (To students in that position, don't do this! Decline offers as soon as you know you won't be accepting, your fellow applicants will thank you!)
In your position I would write an email saying that you are still interested and also indicate that it is among your top choices (as you have said in your question).
Now that you have an admission offer, it is reasonable to contact all (or at least most) of the other schools you have heard from. Any school that you would not attend, email them and say that you have an offer from X and are no longer interested.
Email any school that you want to attend instead of X and tell them you have an offer at X, but would rather attend their school (give a good reason that is not simply their ranking). Tell them the deadline you need to let school X know by. If one of those schools Y gives you an offer and none of the other ones do, you tell X no and Y yes and there are no issues. If a couple of those schools Y and Z accept you, you tell Z yes and X no. You need to tell Y that since the offer you also got an offer from Z that is too good to pass up. This way they will see you were not wasting their time.
The difficult case are the schools you are not sure about. If only school Y accepts you after you tell them about X, then turning them down for X is not ideal. So while it is not ideal, things happen, and no one will hold it against you for long.