When do you know it's time to change jobs

I have been at my current place for over 4 years. It is easy web developer work, and I love the company, it is small (5 people), so the atmosphere is good. I am essentially the lead developer here, but without the pay of one (not even the highest paid developer on the team, ha).

But, I just feel maybe it's time. But it's hard when I do like the company, but recently I think I have come to the realization that you shouldn't treat your job as a relationship, in the end it is business.

I just want to know, basically. When did you know it was time, and looking back on job tranfers what would you have done differently?