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.