GRT remote android app fails to load

I have a Samsung Galaxy S4 I am not using so if figured I would load the GRT Remote Android App on it. The android version on the S4 is 4.4.2 and when I go to GRTs site and download the file, the phone reports a "failure". Numerous attempts have not worked...do I need a later android version?
Ed

My oldest note pad is running Android version 4.2 and not may newer apps will install ... Guessing you will need to upgrade to a newer notepad running a later version of Android as I needed to do.

No, older versions work fine

I have discovered my problem. It was a security issue. It is necessary to go into settings and allow installation of apps NOT found on Play Store. I have it installed now and it is working thanks to clarification from GRT.
Ed