Radical initiation by radical S-adenosyl-L-methionine (SAM) enzymes involves a central organometallic intermediate with an Fe-C bond between the unique iron of a [4Fe-4S]3+ cluster and the 5’C of 5’-dAdo. In some radical SAM enzyme processes, the 5’dAdo moiety is proposed to undergo a reductive-elimination ‘migration’ from the unique iron to a neighboring cluster sulfide, a process that can occur reversibly in synthetic [4Fe-4S] clusters. Sulfide-alkylated [4Fe-4S] auxiliary (non SAM-binding) clusters are further proposed as functional intermediates in a variety of radical SAM enzymes, especially those involved in forming Fe-S or C-S bonds. In this report we use EPR and 13C/1,2H ENDOR to characterize a synthetic organometallic [4Fe-4S]1+ cluster with an iron-ethyl bond, Fe-Et, and its sulfide-ethylated ‘migration’ counterpart, S-Et, thereby furthering our understanding of the electronic structures of these unique clusters and providing a foundation for future studies of enzymatic intermediates involving S-alkylated clusters. In addition, inspired by photo-induced electron transfer from enzymatic SAM-bound [4Fe-4S]1+ clusters, and by Co-C bond photocleavage in B12, we performed 405 nm photoexcitation of both complexes at 12 K. Whereas alkyl-[4Fe-4S]3+ clusters, as well as S-Et, are not photoactive, we find that the alkyl-[4Fe-4S]1+ cluster of Fe-Et undergoes photocleavage of the Fe-C bond at cryogenic temperatures, subsequently undergoing a cascade of H· transfers that parallels the one that occurs during catalysis by radical SAM glycyl-activating enzymes. This observation broadens our understanding of the photoreactivity of [4Fe-4S] clusters as a whole, and of active-site chaperoning of radicals during catalysis by radical SAM enzymes.