What the title says, how do you know what version of plugin to get for what sponge forge version you have installed?
plugin devs should state which version of the SpongeAPI they are for for each version of the plugin and the API version is always listed in the Sponge jar as ...-<API major.API minor.API build>-<build>
As @MoeBoy76 said, plugins will usually state the API version they are compatible with.
However, you should generally always use either the current Recommended build, or the Latest build of SpongeForge. As long as it’s for the same major API version as the plugins you’re using (i.e. 4.x.x), you should’t have any problems.