From Kerbal Space Program Wiki
The Ravenspear Mk4 is a stock plane. It has a simple delta wing airframe with four J-404 "Panther" Afterburning Turbofan. Because of that its performance is not so shocking at high altitudes, but at low altitudes...
|“|| The Mk4 marks a bit of a change in direction from previous Ravenspear models.
Instead of focusing on high-altitude flight, the Mk4 is primarily geared towards making the most out of its four low-altitude turbines.
Being capable of very high-speed flight at low altitudes, it's a favorite among the bravest test pilots, and quite the opposite for the not-so-brave ones.