At the moment of this report, media-libs/coin-4.0.0 depends on an unavailable spidermonkey package slot, dev-lang/spidermonkey:60. See here: https://gitweb.gentoo.org/repo/gentoo.git/tree/media-libs/coin/coin-4.0.0.ebuild#n38 I could successfully build media-libs/coin-4.0.0 against the latest spidermonkey slot,dev-lang/spidermonkey:102, for a test.
Note that the USE flag is masked, which makes this fine. If it weren't masked, such an invalid dependency would never reach users, and would be a severe QA issue. This sounds quite surprising and unlikely. Please share the build.log of a successful build against spidermonkey:102. See e.g. https://github.com/coin3d/coin/issues/173.