If Superman Returns came out 3 years after Superman 2 and had the same director with posters that said Superman 3, it would very much be a sequel.
Bay only labeled TF4 a reboot to avoid franchise fatigue from movie goers. But it's a sequel even more than The Bourne Legacy. Bay isn't fooling anyone.
None of the other TF films have had a number associated with them. Those (teaser) posters seem more indicative of the studio wanting to retain association with their red hot brand than anything else. The early tron legacy media had a 2.0 associated with it, not so much the film campaign itself.
These kinda 'reboots' where a new cast moves forward in the same continuity with the same producers/directors is kinda the way I view StarTrek/PowerRanger franchise reboots. I'm more interested in what something is and it's purpose, than what people think it is. If you move the Tron reboot as is to 5 years after the original and continued that universe with a Bridges covered in aged make up what is now that time as been removed..
Is it time between films? Is it creative heads involved? Is it regard for continuity? Is it cast? Is it characters? Is it title? Is is all of the above...
All these films come with a set of circumstances, how you define them says alot about where you fall in this discussion:
StarTrek 2009
Bourne Legacy
Xmen first Class
Terminator 3
Terminator Salvation
TMNT2007(see canister)
Doctor Who regeneration
StarWars Prequels
21 Jumpstreet
Rise...Planet of...Apes
Prometheus(long time same director)
Highlander2
Highlander3
"Home Alone 4"
Various Zordon involved/non involved power rangers restarts.
Godzilla 2014(see continuity and characters)
Texas ChainSaw 3D(39 years between sequel)
The Wicker Tree
"Carrie 2"
The point being, the definitions seem vague in this day and age.
It's an interesting thing what constitute a 'reboot'. In the world of franchise, reboot is no longer synonymous with 'remake'. Dictionary definitions would assert the word 'revive'. The simple fact is there are various forms or reboots these days and bay himself referred to the thing as a 'soft reboot'. This exact film, 20 years later and with a new director and the issue would be less confusing, but that shouldn't change the truth of the matter.