This thread is a good example of why it's strongly advisable to book through itineraries. Other examples include where trains are retimed, etc.
Note: through
itineraries; not through fares necessarily.
Anyone who books each ticket as if it was a separate journey may run into difficulty if any one of various potential changes that could occur, does occur, and over the years I've seen many examples of this. Admittedly a change of interchange time is rare, but re-timing of trains is not.
If it's literally impossible to obtain a through itinerary, then I would suggest adding each ticket to the basket and pay in one transaction (the forum's site has a basket, unlike the LNER site, which doesn't).
4 minutes at Stevenage is very tight especially if platform 5 is involved. Also with the new timetable in the morning peak the southbound GN stopper arrives exactly 4 minutes before the northbound LNER departs so this is the default connection on most journey planners.
We've already had a thread for that, but if anyone feels that there is insufficient connection time, the forum ticketing site allows you to specify additional interchange time of your choice.