I believe that they are valid to National Rail stations in London not just tube - though I got I think unfairly excesses on this exact type of ticket once, when I was too young to do anything about it. A Cambridge to London 1-4 zones travelcard - got excessed at Woolwich Arsenal and told it was for underground only. Hopefully that won't still be happening to people
You can't get a Cambridge - London Zones 1-4 Travelcard.
An Outboundary Travelcard are generally available from stations in the south-east (although some from further afield are available) is issued as one coupon and allows one journey from the origin station to the boundary of Zone 6 and then unlimited journeys by National Rail, Underground and Bus within Zones 1-6 (there are a handful of exceptions but they are not relevant for the purposes of this thread).
What I think you might have had is a normal return from Cambridge to London Terminals (issued as two coupons) and a separate 'inboundary' Travelcard which are sold in either Zones 1-4 or Zones 1-6 variants. Many websites offer an 'in-boundary' travelcard if you purchase a return to London Terminals. It is rather naughty of them to do this from destinations where an 'outboundary' travelcard is available as an Outboundary Travelcard is almost always cheaper than a return to London Terminals plus a separate 'inboundary' travelcard.
It's also possible that you had a Cambridge - London Zones U1234 ticket. This is not a travelcard. It allows a journey to or from London on National Rail and then one Underground journey to a station in the appropriate zone.
