We offer PHP 5 and Ruby Rails as of early last year. A lot of more technical clients use these two since they either do their own scripting or know how to operate 3rd party scripts.
I think the main reason why many still use php 4 is because they are using outdated scripts that have not been upgraded by the creators or have no technical inclination in these things. We offer php 4 for such people since a lot of people still use php 4, not knowing it is going out the bin soon.
To the OP, this "Install opensource scripts for you" is a bad idea if you plan on doing this for free. Reason being is you will get abused (literally) for more free support on 3rd party scripts that can easily be poorly coded (and some clients do not care to believe this). Another issue is you will be asked for constant free support on these when you know its not a server side issue but an internal configuration error for example. Its not worth taking the risk and responsiblity. You will be overwhelmed and unable to provide support for other clients, unless you intend on hiring people to provide free support on free scripts.
Its one thing to give some tips and advice to the client on such scripts, its another beast to take on the entire responsiblity of doing the work for them. I'm sure your motive is to go the extra mile, but you can easily be worn out in your shoes in such a way like this. This is speaking from 5 years of hosting experience
Free advice never hurts, so I figured I'd let you know.