I was thinking that if I set a flex property and a width property that the width property would override the flex in 4.0.x. Does that sound right? Maybe it's not.

I did see, though, that in the documentation that in order to cancel flex you can use 0 or undefined. Seems in RC3 undefined isn't working to cancel flex, but 0 and null will.

Code:
Ext.widget('viewport', {
	layout: {
		type: 'hbox'
		, align: 'stretch'
	}
	, defaults: { flex: 1 }
	, items: [{
		title: 'Panel 1'
		, width: 100
		, flex: undefined
                //, flex: null
	}, {
		title: 'Panel 2'
	}, {
		title: 'Panel 3'
	}]
});