Download information can be found at Opera Mini 4 beta
Unfortunately is doesn't seem to be supported on the older MIDP 1.0 Java mobile phones.
The current Opera Mini version 3.1 does support MIDP 1.0 so I'd say get it quick and don't lose it.
The new beta version does not appear to overwrite any previous version you had, thus you have access to your previous download, plus the new beta version.
For a minute I thought that the new Opera Mini supported HTML tables and was wondering why it was not mentioned in the features.
Then I realised that it was the CSS support displaying things in a visual manner.
Way cool. (Oh, I'm not talking about people who design web pages using tables. This is tables used as tables.)
gzip is supported (as before) and as Opera states, it'll beat most browsers on download size $$$$$, where most web sites are not natively designed to support compression.
Opera Mini accesses the site you have requested, formats the output and then presents it to your mobile phone.
This is how they can compression data from any site you visit.
Mobile Web browsers (Opera 8.5, Netfront 3.3+, Obigo, Pocket IE, Nokia and etc) used to view sites designed for mobile phones (XHTML-MP) will (at this time) present better than Opera Mini. However most web sites are not mobile ready (more fool them) and older phones web browsers sucked.
Also using your native mobile phone browser to access sites, you may lose compression if both your browser and/or the web site don't support it.
So in summary, Opera Mini 4 looks like it's going to plug a gap in what looks like, a pretty cool way.
Anyway, I liked it.