Why are there no Mach 4+ fighter planes?
Today's fighter are designed with a max speed around mach 2 (max speed is generally calculated with no emport and 50% fuel, the operational top speed is way lower). They keep this ability to pass Mach 1 only to be able to respond fast enough to intercept / air policing missions.

Countries do not design very fast fighters (and planes in general) because :


It's extremely hard to achieve.
It does not give any advantage.
It has no use anymore.


Back in the cold war period, radars and SAMs where not very sophisticated yet, they did not see very far and Ground to Air missiles were not able to get very high. So to protect their airspace from potential intruders / enemy bombers, countries were relying on interceptor aircraft, a fast jet able to catch up the target before it gets to its objective.
At this time, going very fast, very high was an advantage : SAMs where not threatening. The US made a very successful reconnaissance plane exploiting that: the SR-71 Blackbird (max speed : mach 3.3). Many missiles were fired towards the SR-71, none reached its target.

Speed was used for intercept and reconnaissance / intel.

But radars and SAMs improved, getting more range, becoming able to threaten very fast planes. Also satellite images got better and better. The need for very fast interceptors decreased as radar range increased, and the need for very fast recon aircraft decreased as satellite images got better.

Now why does going very very fast not give any advantage to a fighter? Because you cannot turn.
The thing to keep in mind is G force: force the same turn rate, the faster you go, the more g you have to take. And a human can sustain a very limited amount of g. This means a plane going at Mach 4 will barely be able to turn without blacking out its pilot. And if you can't turn, you won't evade enemy missiles, making useless the bit more range gained on the missiles.

And then there is the technical challenge, dealing with a little drag as possible (that means no missile / bomb / fuel under the wings), heat, airframe deformation and wear ... for an advantage which isn't there anymore.

Despite all that, the US are suspected to work on a Mach 6+ aircraft, probably unpiloted.

(Please keep in mind that I simplified a lot of things to not make this post too complex)