I strongly apologise but anybody will not prompt to me !!!
Whether I can use the SP-404 as midi keyboard under Reason???

the sp-404 only has a midi in, meaning it can be controlled by an external device, but it cannot control one

considering the pads aren't velocity sensitive, it wouldn't really make all that great of a controller in any case...

the sp-555 should do it... or an mpd, trigger finger or pad kontrol if you don't need the sampling features