Definify.com

Webster 1913 Edition

Adj.

[

Tetra-

+ Gr. [GREEK] part.]

(Bot.)

Having the parts arranged in sets of four;

as, a

tetramerous

flower

(Zool.)

Having four joints in each of the tarsi; – said of certain insects.

tetramerous ‎(not comparable)

(botany) in four parts: in a flower each whorl (of flower parts) has four flower parts.
As their name suggests, the crucifers have tetramerous flowers.