Four plant growth regulators are: Auxins, Gibberellins, Cytokinins and Ethylene.

Their functions are as follows:

Auxins:

1. Promotes cell division and cell elongation.

2. Causes root initiation in stem cuttings or callus.

3. Promotes apical dominance.

4. Suppresses leaf and fruit abscission.

Gibberellins:

1. Causes stem elongation by increasing the length of the internodes.

2. Breaks seed and bud dormancy.

3. Controls flowering in long-day plants.

4. Induces parthenocarpy.  

Cytokinins:

1. Promotes cell division

2. Inhibits apical dominance

3. Delays ageing in plants

4. Higher relative concentrations of cytokinins (when compared to auxins) in tissue culture medium promotes differentiation of the callus into shoot.

Ethylene:

1. Causes ripeninkg of fruits.

2. Inhibits stem elongation

3. Promotes senescence (ageing)

4. Promotes formation of female flowers in plants like cucumber which bear male and female flowers separately.