You can place Inlets in any direction.
It only needs to be big enough, to feed the engines.
As long as Inlet Efficiency is at 100% everything will be fine. As inlets create much more drag than nose cones, this leads to the ideal placment of inlets backwards behind a nose cone.
Mach 4 with a very simple craft is no problem, when inlet is pointet backwards. Same craft with inlet pointed forwards on the same tank with the same size is only capable of barely Mach 3.3