June 6 (Reuters) - Wisconsin billionaire Samuel Curtis Johnson III pleaded guilty on Friday to misdemeanor sexual assault of his stepdaughter and was sentenced to four months in jail, according to court records.

Johnson, heir to the SC Johnson & Son family fortune, also pleaded guilty in Racine County Circuit Court to disorderly conduct in the case involving the girl.

Johnson, 59, was charged in 2011 with felony sexual assault but prosecutors had to seek lesser charges because they were unable to get cooperation or necessary records from the victim, Racine County District Attorney Rich Chiapete said on Friday.

Johnson faced up to 40 years in prison on the initial felony charges that alleged multiple incidents from 2007 to 2010, starting when the victim was 12 years old.

"I'd like to deeply apologize to my stepdaughter and my wife ... for the tremendous hurt that I have caused," Johnson said during his sentencing on Friday, a video of which was posted on the Racine Journal Times newspaper website.

Johnson will have to serve at least 60 days in jail of the four-month sentence and pay a fine of up to $6,000, according to the plea agreement.

SC Johnson is a privately held Racine, Wisconsin-based maker of household cleaning and other products. Johnson had been chairman of Diversey Inc., a Wisconsin cleaning product company acquired by Sealed Air Corp in 2011, but he took a leave of absence in early 2011, Diversey said in a statement at the time.

(Reporting by Kevin Murphy in Kansas City; Editing by Paul Tait)