Hello Aspirant, here is the seat details of the Presidency's College for Geology-

Geology 17(General) 6(SC) 2(ST) 1(OBCA) 1(OBCB) 27(Total)

So, your OBCB rank is 4 now securing a seat in Geology depends on the luck factor as there is only one seat for OBCB category and if the student with rank 1,2,3 applied for Geology then they will get it.

Hope this helps, if you have any further query pleae let me know in the comments below.