You'll need something to help your body recover from the hormonal imbalance the steroids put your body through. This is the PCT (Post Cycle Therapy) Biggie is referring to.
The best method to get your test production back quick is with a SERM (Different than a SARM; Osta is NOT a SERM). Clomid, Nolvadex, and Toremifene are all SERMs. These help kickstart your HPTA function and limit Estrogen. Along with your SERM you'll also want to run a natural test booster or some kind (D-Aspartic Acid, Ultra Male, etc). At the tail end of PCT it's also beneficial to run an Aromatase Inhibitor (Ai), these prevent test from converting into unwanted Estrogen, some examples are E-Control Rx, Erase, and Eradicate (Same active ingredient as Erase).
You can get by without a SERM, but it's strongly recommended and helps you keep your on-cycle gains. If you don't run a SERM you would just run the Ai and test booster for your 4 weeks of PCT.