I tried with your collection with some changes and it is working fine, see below steps.

Step1: created two variables varText1 and varText2 and use them in collection, Button code.

Step 2: Set Items property of Gallery to colEmptyTab and mapped Index,Col1 and Col2 fields, inserted one more TextInput field and mapped it with Col4. On Right Arrow key add the code.

Step 3:

Step 4: Enter value in TextInput column for col4 and press right arrow button, it will clear collect and save with new values. I just did with one row.