Here is my Dichotomous Key for plants 1,2,3 and 4. 🙂
A. Plant has a flower – B
A’. Plant lacks a flower- Plant 1
         B. Flowers have petals – C
         B’. Flowers lack petals – Plant 4
                    C. Petals are white in color – Plant 2
                    C’. Petals are blue in color – Plant 3
Hooray!!! That is a nice key, straightforward and clean! Well done, Steffi.