Would zinc coated 4043 steel with 17-4 SS produce corrosion? 1

Not if it's dry! Otherwise you have the makings of a battery there.

Also, zinc plated 4340 is susceptible to hydrogen stress cracking (aka hydrogen embrittlement) in a corrosive evironment if the hardness exceeds about 35 HRC, equivalent to a tensile strength of about 157 ksi.

17-4 PH can also be susceptible to hydrogen embrittlement coupled to zinc in corrosive environments.

Thanks for the replays.

So if this assemble is outdoors (on a crane) a rain or humidity will produce corrosion, right?

What could be a simple way to threat the 4340 steel in contact with SS to avoid corrosion while being outdoors?

you question has been answered in part, if it is continuously exposed to an electrolyte(conductive) solution you may have a problem.

defining the degree of concern further definition of your question and possible testings.

ss tends to passivate and reduce the level of concern, but there are a lot of environments that can breakdown the passivation and trigger corrosion.