There is syntax highlighting for Flex 3 and ActionScript 3. For Flex 4 syntax highlighting you have to make only a few changes to this mxml.vim file as follows:

1. Find the line:

syn keyword mxmlSpecialTagName  contained mx:Script mx:Style

and replace it with:

syn keyword mxmlSpecialTagName  contained mx:Script mx:Style fx:Script fx:Style

2. If you have found the above line go five lines down and you will see a sync with mx:Script tag. So you add these one too:

syn region Script start=+<fx:script[^>]*>+ keepend end=++me=s-1 contains=@mxmlScript,mxmlCssStyleComment,mxmlScriptTag,@htmlPreproc
syn region mxmlScriptTag contained start=+<fx:script+ end="+">+ contains=htmlTagN,htmlString,htmlArg,htmlValue,htmlTagError,htmlEvent</fx:script+></fx:script[^>

Alternatively you can download the vim script file from vim.org