I also use a 2 cam set and 5-10 recordings are made per day. I have never deleted anything in 6 months and my 500GB SATA 2.5 inch magnetic disc still has 95% capacity. You need years to fill 4TB. An SSD also brings you few advantages, a magnetic 2.5 inch hard disc is good enough, for example.

Seagate Enterprise Capacity 2,5 HDD ST1000NX0343 (1.02 TB, 2.5")