i think you are stuck.
Without fire OS 5, you can't install the google play store, unless you root the device first. and - checking google play store Hulu app specs - their version also needs android 5+ ( = fire OS 5)
Hulu are not interested in supporting older devices
there is one travel workaround - get a cheap Fire TV stick and take that when travelling - to connect to any tv where there is wifi... cheaper than a new tablet -
alternatively the latest fire HD 8 is regularly on sale at Amazon very low prices , and that could run Hulu ( that or the HD 10 )
a 2016 Fire HD 8 via ebay is even cheaper and has Fire OS 5
We also have a 2012/2013 Kindle Fire 9 inch, in use as a kids tablet, and there's a growing list of unsupported apps to its age. but it's still good enough to keep grandchild amused
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