Hi @chengjh1 and thanks for your query. Unfortunately, the light settings are not adjustable on the 4C and are a necessary component for correct eye tracking so any attempt to physically cover them may reduce accuracy and trackability considerably.
Our apologies for the inconvenience, it may be worth further illuminating the study room to reduce the contrast between the illuminators and the stimulus.
Please kindly let us know if we can provide any further information to help you setup your project.