Paul Scott wrote:

Sure? Doesn't the ticket being a season negate the requirement for an actual
stop at the changeover point?

Condition 19 of the National Rail Conditions of Carriage applies:

-----Start of quoted text-----

*19. Using a combination of tickets*

You may use two or more tickets for one journey as long as together they
cover the entire journey and one of the following applies:

(a) they are both Zonal Tickets (unless special conditions prohibit
their use);

(b) the train you are in calls at the station where you change from one
ticket to another; or

(c) one of the tickets is a Season Ticket (which for this purpose does
not include Season Tickets or travel passes issued on behalf of a
passenger transport executive or local authority) or a leisure travel
pass, and the other ticket(s) is/are not.

(rest snipped)

-----End of quoted text-----

For Condition 19(a) to apply, you'd need a Potters Bar - R56 Travelcard
and a Zones to 4 Travelcard.

Condition 19(c) doesn't apply as a Travelcard is *NOT* a Season Ticket
or Travel Pass issued on behalf of a PTE or Local Authority.

Retail Manual Part 1 gives the following examples:

Example 1: A customer has two Season Tickets: One between Northampton
and Bletchley; and the other between Bletchley and Watford Junction.
They may only travel from Northampton to Watford Junction on trains that
call at Bletchley.

Example 2: A customer holding a Northampton to Berkhamsted Season Ticket
and a Berkhamsted to London R1256 (=AII Zones) Travelcard Season Ticket,
may only travel between Northampton and "London R1256" on trains which
stop at Berkhamsted.

Example 3: The holder of an Annual Gold Card Season ticket between
Manningtree and Witham wishes to travel from Manningtree to London
Liverpool Street. A ticket from Witham to London Terminals is required
to make this journey. This may be purchased with the appropriate Annual
Gold Card discount if applicable at the time of travel and is valid on
any services operated by "one". The chosen train need not call at Witham.

Example 4: The holder of a 7 Day Freedom of North East Rover ticket
wishes to travel from Kings Cross to York. A ticket from London
Terminals to Retford is required to make this journey. The chosen train
does not need to call at Retford.

Example 5: A customer with a three months Travelcard Season Ticket
between Sittingbourne and "London R56" ("Out Boundary" to Zones 5 + 6)
wishes to travel for one week only to Beckenham Junction, in Zone 4. To
do so, they must purchase a 7-Day Travelcard Season Ticket for Zone 4
and use both tickets for the through journey. It will not be necessary
to change trains if a through service is available.

Example 6: A customer with a three months Travelcard Season Ticket
between Brighton and "London R56" wants to travel for one day only to
Romford, in Zone 6, on the opposite side of London. To do so, he must
purchase appropriate tickets to enable him to cross Zones 4, 3, 2 and 1
into London and then zones 1-4 between London and Romford. Accordingly,
a Peak or Off-Peak Day Travelcard for Zones R1234 to be used in
conjunction with the Travelcard Season Ticket would need to be purchased
before travel commenced.

HTH,

Barry