for ipad air 4th gen, will it support 45W charger ?

Yes, you can safely use the higher wattage Power Adapter with your iPad. You should note that the iPad will only charge at the maximum supported charging rate supported by the iPad.

iPad Air4 supports USB-PD Fast Charging. When connected to a USB-PD compliant Power Adapter, such as the Apple 45W Adapter, your iPad will charge at its maximum supported charging rate - this being 18W (1.5A @12V). When charged by a standard USB Power Adapter, charging is limited to 12W (2.4A @5V).

Any iPad can support any Apple power/charging adapter blocks , but any iPad model will only take the power it needs to charge

Your new 2020 iPad Air 4 will probably only take up about half that 45W charging capacity.

Yes, you can use that or even larger power adapters if it has a cable with a USB-C connection.