If the pattern or the logic of selecting the number is in this way - The Two digits of the First 4 digits found is supposed to be removed from the data.
Then, we could Check with the Regex Replace in the below way :
(new System.Text.RegularExpressions.Regex("\d{2}(\d{2})")).Replace(inputText,"$1",1)
Here, InputText is a string type variable containing your input data.
Visuals from Debug :
Let us know if there is a more specific logic that you would need to apply for the removal of that particular data.