If only one Norman twin was running for this,
there would be 9 from which to choose 4.
If both Norman twins are running and they are distinguishable,
(different clothes, different hairstyles etc),
then there are 10 available from which to choose 4.
Therefore the 2nd twin introduces an extra possible arrangements.
If they decide to be indistinguishable, then when 1 of them is in the group,
the other can swop places undetected.
This makes them effectively a group of 2 indistinguishable members, for all the arrangements either one is in.
When they are together, 2 of the 4, indistinguishable, they are effectively an identical group of 2.
Hence we divide the added arrangements contributed by the 2nd twin by 2!
Therefore, the number of arrangements is