You would need to case the infoobject as IWebi instead of IReport as IReport is an interface to reference crystal reports. Please refer to the dev guides available at help.sap.com for understanding the SDK's better.
Below is the example how to do it
IWebi boReport = (IWebi)boInfoObjects.get(0);
IWebiFormatOptions boReportFormat = (IWebiFormatOptions)((IWebiProcessingInfo)boReport).getWebiFormatOptions();
//Set report format.
int boFormatType = IWebiFormatOptions.CeWebiFormat.EXCEL;
boReportFormat.setFormat(boFormatType);
Also if you are on BI 4.x version, you would not be able to schedule webi reports with prompts using the BusinessObjects Enterprise Platform Java SDKs, however simple schedule is still possible.
You would need to go for the Restful webservice sdks for that. Search on the scn forum and you will find a lot of posts which would explain why you cannot schedule a webi report with prompts from BI 4.x
For any queries regarding Restful API's refer to the below forum. It has many blogs and documents which might help you
scn.sap.com/community/restful-sdk
Thanks,
Prithvi
