here is a short list of shot sizes and the equivalents you could use, you can see just by looking at the chart that a BB = 2 x No4 so if you wanted to spread that even further you could use 4 x No8s or to take it to the extreme you could even swap 1 x SSG for 16 x No6s (but thats just an example of how to use the chart)
Split shot size......Weight....................Shot equivalent
SSG....................1.60g .....................2 x AAA
AAA.....................0.80g.....................2 x BB
AB.......................0.60g.....................2 x No1
BB.......................0.40g.....................2 x No4
No1......................0.3g......................3 x No6
No3......................0.25g....................2 x No6
No4......................0.20g....................3 x No9
No5......................0.15g....................2 x No8
No6......................0.10g....................2 x No10
No8......................0.06g....................2 x No11
No9......................0.05g
No10....................0.04g....................2 x No12
No11....................0.03g
No12....................0.02g....................2 x No13
No13....................0.01