For lookup references to other entities you need to specify both the GUID and the type. You also need to include the UOM when creating an Opportunity Product based on an existing Product. Your object should look something like this:
var entity = {};
entity.Quantity = parseFloat(4).toFixed(2);
entity.OpportunityId = {
Id: '69BB2236-B57F-E311-BB6D-6C3BE5A881A4',
LogicalName: 'opportunity'
};
entity.ProductId = {
Id: 'C8138483-DF81-E311-B542-6C3BE5A8362C',
LogicalName: 'product'
};
entity.UoMId = {
Id: 'BE0FB859-7E90-4B3E-B501-3AB3CD4DC8FC',
LogicalName: 'uom'
};