Season ticket refunds are calculated using a base rate which is the monthly season price divided by 3.84, rounded to the nearest 10p. The base rate is then used to calculate differing season ticket periods by multiplying that base rate by a factor determined by the amount of usage the ticket holder has had from their Season. The base rate should be the prevailing monthly season/3.84 available at the time the monthly or longer season was purchased. Season ticket refunds are also subject to a ^10 admin fee.
Table for calculating multiples of base rates for season ticket periods from 1 month to 1 year (extract from NFM98):
So, if you return your season ticket after 4 months exactly the refund will be: ^4516 minus (base rate x 15.36) minus ^10 admin fee. The base rate for your season is ^112.90.
Should you need an annual season for a different journey part way through your current season's validity, then a changeover rather than a refund and re-issue will be better value. You can do a changeover in any circumstance, provided the season has a month or more remaining. A season changeover is worked out with a daily rate for each ticket. This daily rate will be 1/365 of the price of the current and new annual seasons. The price of the current and new season (from the original start date to the expiry date) will be divided by the total number of days the current ticket is valid for. Those figures are then multiplied by the number of days remaining on the ticket. The difference between those figures is refunded (or charged as applicable).
I believe ticket clerks don't have to perform these calculations themselves, their Ticket Issuing System does it for them.