This is a testosterone injection consisting of a mix of propionate, phenylpropionate, isocaproate, and decanoate esters. Because of the mix of different esters, "4 Test Inj" has improved pharmacokinetics with the fast action of propionates and the prolonged action of longer isocaproate and decanoate esters, thus providing a more stable concentration of testosterone in the blood over a longer period of time. Esterification of the 17-beta-hydroxy group by different esters can prolong of action of testosterone, because testosterone esters are less polar than free testosterone. Testosterone esters in oil injected intramuscularly are absorbed slowly from the lipid phase; thus "4 Test Inj" can be given at intervals of two to four weeks.