Assuming you have access to color, and vertical space isn't tightly constrained...
I would start with the excellent 4e stat block.
Adapt this to the other systems. The basic template goes like this:
Colored title bar, including creatures name, level, and type information. You should include the name of the system up here as well.
End-use derived combat-relevant stats (light background color).
Attack modes/special abilities (Description of attack in dark background color, details of the attack in light color).
Full statistics not listed elsewhere (dark background color).
Finally, give each system a unique color (this will lose some fidelity in that I think 4e uses that color coding, but shouldn't be a big deal).
This will give you a stackable set of color-coded stat blocks. Playing Savage Worlds? Look at the red stats. 4e? Blue.
(Note: this will work for traditional RPGs, but the ones that are way out there might not fit. I'm not familiar with FATE or Savage Worlds, so I can't be certain it'll work for those.)
Combining Stats on the Same Line
It was pointed out elsewhere in this thread that what you're really looking for is collapsing everything to the point where the statistics from the different systems all share the same line within each category (i.e. a single defense line with notes from 4e, 3.5, SW, and FATE).
This is a much more difficult proposition (particularly when scaling up to 5 systems), and may not be solvable.
The primary problem you're going to have is quickly acquiring the stats for a system other than the first one in the list.
The issue is that there's no way to tell where a specific system begins without reading everything prior to it (which may mean trying to read game stats for a system you don't understand). It also becomes much more difficult to get an "at a glance" overview of the creature, because you must scan through other systems.
Another (less important) issue is that you will have to work a bit harder on the formatting to avoid the "wall of text" scenario that stat blocks are intended to avoid.
A few potential solutions:
Using color can help, but having many different colors interweaving that closely may become confusing. Your color selection will also be limited to colors acceptable as body text, and you won't be able to utilize .
Prefix each system's subcomponent with an icon indicative of the system. It's still going to be a bit of a hunt to find all of the icons, but it's a bit faster than needing to scan the text itself.
Be very conscious of your whitespace. You'll need extra, because you have so much text trying to share the same space.
Type up the name of the monster and its overall description in the body text of the module. Then use its stats as a sidebar (vertically stacked). If these sidebars get to be too tall (a real possibility with four or five systems!) you can stack them horizontally. If you do, I recommend separating them with with a black border, or a border that matches the title bar's color.
Pick a unified design that can be applied with reasonable modification to all of the systems involved.
Use something visual that can quickly allow the reader to zero in on the stats they want. Color is probably best for this, but might not be available in all formats (i.e. when publishing a black and white book).
Push generic fluff outside of the stat-block itself, so you can keep the stat blocks compact.