RCS-4 [(4-methoxyphenyl)-1-yl-(1-pentyl-1H-indol-3-yl)methanone] represents the first of several N-alkyl-3-(methoxybenzoyl)indoles identified by forensic scientists as synthetic cannabinoid (SC) designer drugs. Despite the detection of RCS-4 and several analogues (RCS-2, RCS-3, RCS-2-C4, RCS-3-C4, and RCS-4-C4) in products intended for human consumption, relatively little is known about this class of cannabinoids. The synthesis of all regioisomers of RCS-4 and their C4 homologues is described. This study also systematically explored the structure–activity relationships of this class of SCs at human CB₁ and CB₂ receptors using an in vitro fluorometric imaging plate reader membrane potential assay. All compounds demonstrated agonist activity at CB₁ (EC₅₀ = 54–574 nM) and CB₂ (EC₅₀ = 4.5–46 nM) receptors, with the C4 homologues showing a preference for CB₂ receptors over CB₁ receptors (31–42 times). Since most of the analogues (RCS-2, RCS-3, RCS-2-C4, RCS-3-C4 and RCS-4-C4) are not subject to regulation in much of the world, despite their activities towards CB₁ and CB₂ receptors, there is a possibility that these analogues will emerge on the black market.