I don't really have an answer for you, but wanted you to know that you are not alone. I am in the exact same boat as my 1st dd is 4 1/2 and in preschool. I find that driving her to and fro negatively impacts dd #2's ability to really nap. She gets lulled to sleep a bit too early b/c of the motion and it's making naps a nightmare. She'll only nap for 35 ins. Occasionally, I can get her to extend the nap by resettling her, but that is touch and go. My other dd needs me too, and I can't stand around and resettle dd #2 for 25 mins.
Eventually, dd#2 will be able to stay awake longer and her naps will get longer. I've done my best and am now just deciding to accept what I've got.
By the way, have you tried "topping off" your lo right before her bed time? I generally stick to the 4 hr easy for my 5+ mos old dd, but she is hungriest in the evening so I feed her around 6 (a small bottle 2-3 oz) and cereal/fruit. Then at bedtime, 7:30 ish, I nurse her (in another room so that she is not being "nursed to sleep"). My lo also gets a dreamfeed at 11pm. As a result, she usually sleep thru til 7am. Do you do the dreamfeed? Have you considered topping her off before bed?
Best of luck to you.