Is that BSL you're going to?
If so it's very helpful to have a part number handy.
For a Simrit/Freudenberg simmerring 4mm shaft seal-
BA 4,00 12,00 6,00 72 NBR 902
I'm sure the 4mm seals I have are 11mm outer diameter, so they seem to have changed the spec to 12mm. Not sure why.
They also do a larger seal of 16mm outer diameter, also suitable for 4mm shaft-
BA 4,00 16,00 7,00 72 NBR 902
I prefer the smaller version, but the larger seal is a bit less fiddly.