I believe you're fundamentally mistaken about the 5 segment SRB because you're assuming that they're simply going to add another segment without changing anything else. It all comes down to the propellant geometry and the amount of exposed surface area during the burn. If you simply add another segment without changing the propellant geometry, then at the moment of ignition, you'll be exposing 25% more propellant face to combustion. This additional burn area will result in higher internal pressure (perhaps to the point of explosion if the casing can't withstand the pressure) but also higher thrust. However, if you modify the propellant geometry, you can get any number of thrust profiles. For example, if you leave out the hole for the 5th segment, you'll get a lower thrust for a longer time. Or, instead of a star geometry, you can go with a simple circle through all 5 segments. This would result in a high initial thrust with increasing thrust through the burn as more surface area is exposed. Solid rocket motor design isn't new. Engineers know how to tailor the geometry to get the desired thrust profile for the mission.