Excellent! Finally find this thread that answers my question. Download extjs4.1.beta-v0.1.sdocml you can add code hint to your ExtJS 4 Aptana project.
There are many instructions for adding the Spket plugin to Eclipse / Aptana to support ExtJS. But it just never works for me ... Not sure if anyone used it and developed it.
UPDATE I use WebStorm to develop ExtJS 4, it has very good ExtJS support. In fact, all js files included in the project can be identified in WebStorm, and thus a hint is provided.
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