Not to mention that for 4g acceleration the thrust in lbf needs to be at least 4 times the MTOW in lb of the aircraft (not even counting drag...).
Seems overkill to me.
tartare, where did you get this notion?
And BTW, LAPCAT is an early study of advanced propulsion techniques, not an aircraft design.
The Reaction Engines A-2 is a paper exercise to give an idea what such an aircraft
might look like in 25 years time.