There can be lot of syntactic or other DIFFERENCES between the two, but major difference is: In Flex 4 the architecture of components (most of) have GOT changed. These components (called Spark the older ones in Flex 3 are called Halo) have separated the role of developer and designer. Spark components have one main CORE component class (written in actionscript) that contains the main LOGICAL part and one Skin class that HANDLES all the visual aspects. We can say core component is skeleton and skin is its visual appearance. For example spark button has one core Button class and one skin class for it.

There can be lot of syntactic or other differences between the two, but major difference is: In Flex 4 the architecture of components (most of) have got changed. These components (called Spark the older ones in Flex 3 are called Halo) have separated the role of developer and designer. Spark components have one main core component class (written in actionscript) that contains the main logical part and one Skin class that handles all the visual aspects. We can say core component is skeleton and skin is its visual appearance. For example spark button has one core Button class and one skin class for it.