Are you using the “text” editor tab when posting your iframe code or are you trying to post the code in the “visual” tab?
Doing a google search using search string: “wordpress 4.0 text editor strips out code” I came across this topic: http://wordpress.stackexchange.com/questions/147303/wordpress-visual-editor-stripping-html-changes
I just tested some basic iframe code and it worked fine and was not stripped out of the text editor
<iframe src="http://example.com/" width="200" height="200"></iframe>
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