Return on ad spend (ROAS) shows how much revenue comes back per euro of advertising. A ROAS of 4 means: for every euro of ad spend, 4 euros of revenue flow back. Sounds good — as long as you do not forget the contribution margin.
The break-even ROAS is exactly the value at which advertising spend is just covered by the margin. At a 25 % margin it is 4. Meaning: a ROAS of 4 is not profitable but zero — anything below makes a loss. That is why POAS (profit on ad spend) is the more honest metric when the margin is known.
Watch the attribution window: last-click overestimates performance channels, view-through underestimates classic advertising. For strategic decisions, a comparison with ROMI over several months is worthwhile.