hi will - if i had to throw out a guess, it is that the ext4 client API isnt being loaded on your page. it is probably best practice for your code to explicitly declare that dependency. i cant say with certainty, but it is possible some other piece of code that happened to get loaded on your assay page was requesting this library, which meant it was already available for your code.

if you're doing a file based assay, i would recommend also creating a file named upload.view.xml. you will want to add something like:

<view xmlns="http://labkey.org/data/xml/view">
    <dependencies>
         <dependency path="clientapi/ext4"/>
    </dependencies>
</view>

check out similar upload.view.xml files in the modules under /externalModules/labModules/