For Boeings, the 747-4 or 767-3 specifies the model number, ie 747-400 or 767-300 in the case. The last 2 characters specify the customer number. The customer fit will vary in some way from the manufacturers basic specification.
So, BA has customer number 36, so BA's 747-4's are B747-436 etc.
HTH, b ut a search would reveal previous discussions - btw, Airbus do it differently!