Eric Rafferty, 69, of Aberdeen, was convicted of seven offences of rape, attempted rape and indecency against his victims between 1982 and 1990.
Judge Lord Uist told Rafferty at the High Court in Edinburgh: "These are offences of the most appalling nature.
"It has taken a long time for justice to catch up with you, but the day of reckoning has finally arrived."
He added: "All four girls were subjected to the most abhorrent sexual practices by you. These were unspeakably wicked crimes."
Defence solicitor advocate Iain Paterson said Rafferty partially accepted responsibility for the sex crimes but denied rape.
Mr Paterson said that if Rafferty survived prison he would come out "significantly older".
He said the former carer had no previous convictions, but that Rafferty accepted a custodial sentence was "absolutely inevitable" and would undertake a sex offender programme in prison.
Lord Uist said: "The only reason that it is now he is being dealt with was because, as is fairly common in this type of case, a sufficient number of victims did not make their disclosures until many years later."
A child rapist whose abuse of young girls was described as "wicked" has been jailed for 15 years.