If losing a drag race to one of your siblings is more acceptable in the automotive industry than being out-performed by a direct rival, then neither of these cars has anything to worry about.
On one hand, it would take a brave M3 owner to want to challenge an M4 GTS, even if it's just a straight line drag race and not a quick lap of some racing circuit - which is where the M4 GTS shines.
Then again, with the Competition Pack, the M3 truly is at its best when it comes to acceleration. It's got 450 PS (444 HP), better wheels, plus upgraded springs, dampers and anti-roll bars for its Adaptive M suspension.
As for the M4 GTS, not only does it have 500 PS (493 HP) and 600 Nm (442 lb-ft) of torque, it's also missing its rear seats, boasts an enhanced M differential, a re-tuned electromechanical steering system, adjustable suspension and a massive rear wing, which is probably one of the first things you notice when looking at it.
In the end, the race was quite close, but mostly because one of these two M-cars struggled to get decent grip off the line.