Answer:
0 additive alleles = A0B0C0D0
1 additive allele = A+B0C0D0, A0B+C0D0, A0B0C+D0, A0B0C0D+
2 additive alleles = A+B0C0D+, A+BC+D0, A0B+C+D0, A0B+C0D+, A0B0C+D+, A+B+C0D0
3 additive alleles = A+B+C+D0, A+B+C0D+, A+B0C+D+, A0B+C+D+
4 additive alleles = A+B+C+D+