Incomplete linkage refers to the situation where two genes are located on the same chromosome, but they are not tightly linked, and some recombination occurs between them during meiosis.

Possible gametes produced by a dihybrid plant with incomplete linkage on test cross are:

- AB
- Ab
- aB
- ab

Each of these gametes has a 50% chance of being produced.

When these gametes combine with gametes from the tester parent which is homozygous recessive (aa bb), the resulting offspring will have one of the following genotypes:

- AaBb (phenotypically dominant for both traits)
- Aabb (phenotypically dominant for the first trait, recessive for the second)
- aaBb (recessive for the first trait, phenotypically dominant for the second)
- aabb (recessive for both traits)

Therefore, there are four possible genotypic combinations in the offspring. However, each of these combinations can occur in two different ways, depending on which gamete came from the dihybrid plant.

For example, AaBb can be produced by the combination AB x ab or by the combination Ab x aB.

Therefore, there are eight different phenotypic combinations that can be produced in the offspring:

- AaBb (AB x ab, Ab x aB)
- Aabb (AB x ab, aB x Ab)
- aaBb (ab x AB, aB x Ab)
- aabb (ab x ab, Ab x aB)

Hence, the correct answer is option B (4).