There are some differences between normal widgets and mobile widgets you should be aware of. In short:
In order to be able to use a widget on a mobile form, a widget should be defined as being a mobile widget in the xml definition:
<widget id="custom.widget.HelloMobileWorld" mobile="true">
The mobile user interface is defined in its own namespace: mobile. Everything defined in the mxui namespace is not available on mobile. Furthermore the 'mobile client' uses a different user interface subsystem (mx.ui) which defines a different API in mobile.
P.S. Mendix 4.0 uses version 1.6.1 of the Dojo Toolkit.