Ah sorry, I misunderstood. Do you have 4.8 preview 4 installed? If not, it may be letting you know that you can add an engine slot for the 4.8 preview builds (or if you already have it but have preview 1…3, that preview 4 is available). If you already have 4.8 preview 4 installed, then I have no idea why it’d be telling you there is a new build available.
Cheers,