Adding a fourth weekly run when three days feels sustainable

Three runs a week fit my life and leave me reasonably recovered. I'm tempted to add a short fourth run for more consistency, but I don't want to convert a sustainable hobby into a schedule I resent. Would you make the new run ridiculously easy—twenty minutes at conversational pace—or distribute the existing weekly distance across four days first? There's no race deadline; I'm just enjoying getting fitter.