I would expect it to be present for most versions, but if I were you, I'd write in a safety check, perhaps adding a script at installation time to check for presence of the Microsoft.Jet.OLEDB.4.0 and downloading or installing it if necessary. You should be able to distribute this as a part of MDAC package and there may be other way to redistribute the provider in a small package. For example, I know that Access 2007 and 2010 has a redistributable edition (this is not same thing as runtime)... 2007 call it "Access Connectivity Engine" and 2010 is just "Access Redistributable", IIRC. I don't think there was such editions for pre-2007 so you may have to use MDAC to redistribute the 4.0 rather than 12.0 provider. (Do note that 12.0 provider ought to work just as well with 4.0)
Prior to 2007, Jet was basically a Windows component, distributed in various channels (MDAC, Visual Basic installation, SQL Server installation, etc., etc.) and I'm pretty sure that has been since Windows 95 or 98 so it's very unlikely you'll find a Windows that didn't have Jet already installed somehow, but why take the chance?
HTH.
