Just a little addition here, if you're thinking about getting a K&N filter to replace the stock filter in your metal HS4 case,
don't use the one stocked/recommended by all mini shops.
Standard filter = GFE1008
OD = 210mm
ID = 160mm
Height = 55mm
Mini shop reccomendation = KN E-2601
OD = 187mm
ID = 156mm
Height = 56mm
Proper filter = KN E-2600
OD = 208mm
ID = 159mm
Height = 56mm
The metal HS4 case locates the filter using the external lip, which is 210mm. If you use the recommended E2601 filter it will be loose inside and can block the carb intake. This is the filter pushed towards the front of the case

And pushed to the back, you can see the smooth shiny metal where the filter has been rubbing

The E2600 is a perfect fit for the metal case

The E2601 would be a good fit for a plastic air filter case as the filter locates using the inside diameter and the smaller outer diameter doesn't matter
