@Anonymous
You need not worry about the Textbox to do anything. In PowerApps, things "respond" to other controls, controls don't usually alter other things.
So, looking at your original default formula for your Gallery Items Property is the place to start.
You can see that it had two functions in it - the SortByColumns and the Search function. You will need to apply those to your Gallery Items property as well.
Consider altering this formula for your needs:
SortByColumns(
Search(yourDataSource,
TextSearchBox4.Text,
"yourColumnYouWantToSearchInQuotes"),
"yourColumnYouWantToSortByInQuotes",
If(SortDescending1,
SortOrder.Descending,
SortOrder.Ascending)
)
This will give you similar functionality for searching and sorting that you were asking for, but you need to apply your datasource and your columns.