Hi Ric.
A couple of questions - what type of wash do you have? IBA, friction/touchless, mini tunnel?
A couple of things to consider. Towels to dry windows, mirrors and chrome - must be laundered. You may also want to use an extractor type machine so have them slightly damp. So now you have some machines/detergents to buy and a few more chores for your attendant to keep up with.
Tire shine - if you have an IBA or even a tunnel that is only spraying the tires, you will have a brown/black sludge on your applicators and customers tires. Tires getting tire shine need to be very clean before they receive this service, or they look terrible and your applicators become a mess.
Also as an aside - I hate to see tires with tire shine unless the wheels are clean too. So if your wash process is not one that does a decent job on brake dust, it is kinda like wearing a new suit with old, dirty scuffed shoes.
Interior towel (damp) - This will work for some of your customers, but there are dash boards, interior trims, jambs etc, that are caked with dirt. Unless you are using some type of cleaner, you will just be turning this to a wet mess. Also - where will your attendant draw the line - will he/she do steering columns, the foot areas, jambs. Dash boards only? What will the expectation be?
You will have to have quite a few more tools at your disposal to do this effectively. I know you would only charge $4 for the service, but if it isn't feasible do a quality job, you won't have many takers anyway, and the whole thing is a moot point.
I certainly do not mean to tell you this is not a good idea - please do not misunderstand - I just know (from personal experience) that there is always a lot more to things like this than I initially realize.
Good luck and keep us posted!
Mike