Easiest way to tell Davies obv 4 from Davies obv 5 is the tip of Victoria's bun. Davies obv 4 ends in a closed loop, Davies obv 5 ends in an open hook as per 1st picture.
1867 is the only year for which both obv 4 and obv 5 exist and there are two reverses, so 4 combinations; 4+A, 4+B, 5+A and 5+B.
DN 16 & 17 are 5+B, DN 18 is 4+B, DN 23 is 5+A, all the rest are 4+A.
The 2nd picture shows Davies obv 6 and obv 7. Both bun tips are closed loops but slightly different shapes.
The Spink designations are Davies obv 4 (S.3904 & S.3905), obv 5 (S.3906A), obv 6 (S.3907A), obv 7 (S.3907).