Speed control of 30 amp, 90 volt, 4 hp DC treadmill motor
The controller's current capability must meet or exceed the current requirement of the motor. If it exceeds the current capability, the price may be higher than it would have been if you had been able to find one with a 30 amp rating. The efficiency may be a little lower. If the current capability is too low, the motor may not start reliably or it may have difficulty maintaining speed under load.

The controller's voltage capability should be as close as possible to 90 volts. If it is too low, the maximum speed of operation will be less than it is supposed to be. If it is too high, it will be possible to operate at a higher speed than the machine is designed to operate. If you attempt to operate at higher than the design maximum speed, more than the design power will be used and the motor or some mechanical part may overheat. The controller may have an adjustment to limit the speed. That would prevent the problem. In that case, the only disadvantage would be possible reduced efficiency and higher price.

There is quite a bit of flexibility in meeting the 30 amp, 90 volt specification with only a little risk of difficulty.

You should also consider the momentary and short-term current rating. Built-in current limiting and electronic over-current trip features should also be considered.