For a given metal ion, weak filed ligands create a complex with smaller $$\Delta $$, which will absorbs light of longer $$\lambda $$ and thus lower frequency. Conservely, stronger filled ligands create a larger $$\Delta ,$$ absorb light of shorter $$\lambda $$ and thus higher $$v.i.e.$$ higher energy.

$$\mathop {{\mathop{\rm Re}\nolimits} d}\limits_{\lambda = 650\,nm} \,\,\mathop { < Yellow}\limits_{570\,\,nm} \,\,\mathop { < Green}\limits_{490\,nm} < \mathop {Blue}\limits_{450\,nm} $$

So order of ligand strength is

$${L_1} < {L_3} < {L_2} < {L_4}$$