Okay, I only have my DS (19 months) so I don't have any experience with an older sibling interfering with naps. But, here are my thoughts:
Is he tired? My DS always seems to be tired for his naps as long as I wait until at least noon (after lunch). If I try earlier than that he won't actually fall asleep and it will mess everything up for later. He only has one nap a day and has since he was about 12 months.
Do you nurse? DS always nurses to sleep for naps, I think because he's so tired. Bedtime is a whole other story, unfortunately!
Family bed? I have this (possibly naive) image of a you laying down with the 1 yo, while the 4 yo snuggles/looks at books/other quiet activity next to you. It can be your family 'quiet hour' but at least the 4 yo gets to stay with you. Maybe it would work?
Mama to my charming little boy, born at home January '09