Now this is a wild theory but it is possible(correct me if i’m wrong at some points.
It also goes with this theory,which you have to see it first before reading my theory. thread and video video
So after master chief is saved by unsc infinity and dock with it by putting an air pressure seal his Mark 6 suit is to damaged to wear from explosions so he takes it off leaving it on unsc forward unto dawn.
So once master chief enters infinity they let go of the forward unto dawn leaving it to go to the shield world. Master chief then receives mark 7 armor. Then the reclaimer trilogy happens. leaving master chief to go back to shield world at the end of halo 6(reasons unknown or whole trilogy reovles around the shield world)His mark (by halo 6) 8 or 9 is damaged and has to escape it before shield world explodes and finds the forward unto dawn and puts back on his mark 6 and uses some sort forerunner technology around 5:50 and gets lost in space again,then halo 7-9.
This would explain why cortana says she is 7 years old even though halo 4 takes place in 2553.This would also explain why she knows this 2:46 Til the end or this could be her rampancy.
Sorry for any grammar mistakes. Let me know what you think.