Your iPad Pro M4 will not charge any faster if using a 70W Power Adapter.
While Apple has not released a definitive specification for the maximum charging rate, most models of iPad Pro charge at their fastest when charging from a 20W USB-C Power Adapter that supports USB PD (Power Delivery). You should note that to protect the Li-ion battery, the maximum charging rate is significantly reduced above 80% of maximum charge.
Higher output Power Adapters provide a higher "power budget", the additional power capabilities being useful in providing additional power for connected USB devices and other accessories (such as a USB-C hub, USB storage and Keyboard) while maintaining the ability to charge a discharged iPad battery at its maximum rate.
The iPad's maximum charging rate can be maintained using either the Magic Keyboard's USB-C pass-through charging port and the Keyboard's Smart Connector, or the iPad's own USB-C port.