steel for a hammer?
 

what kind for a japanese style forgeing hammer?

4140 is great stuff for hammers. I have also used 5160, 52100, and 1050. It's all in how you heat treat them. I like 4140 and 1050 for softer hammers, which I use on fine work. 52100 and 5160 better for harder hammers for general forging and for making larger hammers out of.