Hi there and welcome to the forum. Would it be possible to see a shot of the complete flag please (with dimensions if possible)? This type of corner patch would have been found on early HJ Gefolgschaft flags. The numbers stand for:
(Gefolgschaft) 4 / (Unterbann) II / (Bann) 288
This flag was in use no earlier than 15.5.1934, this being the date on which Bann 288 was raised. However, this kind of numbering convention was deprecated in Feb 1935 and replaced with, in the case of your unit, 4/288. This change to unit numbers was quickly followed by the introduction of standardised flags with different corner patches showing that new numbering convention so in your case the panel would show 4/288 (black stitched numbers on a white panel). You can see an example of this type of flag
here.
As far as the location of that unit is concerned, our wiki page
here shows that Gefolgschaft 1/288 was located in the town of
Eltville. The Bann headquarters was also located in Eltville and given the size of the town (population 4400 in 1935) I suspect that Gefolgschaft 4 was also located there or very near. Bann 288 (lists can be found
here) was called Niederwald at the time when your type of flag was in use. This was in HJ
Gebiet 13, Hessen-Nassau.
Note: the first and third links in the above paragraph lead to Full Member sections of the forum so you won't currently be able to open them.