You have root on your server, you can install them if you like. There are several tutorials on how to run both php4 and 5 on the same server. But we won't support it now as it requires a lot of custom work.
You'll have to run php as a cgi when you run both versions I believe (if not both, certainly php5 is run as a cgi)
MySQL 5, not anytime soon either. [EDIT- MySQL 5.0 upgrades are being completed the week of August 13th, 2006)
We probably will not offer it on production servers until they are compatible with cPanel which is in beta now.
We will likely add a development server that has MySQL 5 and PHP5 and a possibility of Apache 2, and perhaps some other technologies.
That should satisfy some demand by the dev community until we see it as an option for the production servers.
We will make an announcement when we have a dev box online. If anyone would like to reserve a reseller account on this machine please PM me for a 20% discount. When we make plans available we will notify you and if you are interested you can use your 20% coupon.