So in the Imp Companion description the Imp's level is based on the Diabolist's classLevel + Highest CasterLevel. So in order to get the imp's 4th level stat bonuses, alternate form, and SLA's do you need to reach Diabolist class level 4? or does the imp already have these boosts because by the time you gain Diabolist level 1, your casterlevel is well above the required 4th level?
your imp already has the bonuses, you've attracted a fairly high ranking Imp to your side.
at least that's what the rules are saying.