OK, that makes more sense!
My LO did 4 hours of daytime sleep for quite awhile too -I think up to 6 months. It was suggested on the nap board that I shorten his daytime sleep b/c we were having bedtime issues. That was a disaster! Probably my solution should have been extending awake time before bed. My LO shortened his naps on his own really quickly after the 6 month mark. Suddenly we went from 4 to 3.5 then 3, all within about 2 weeks. You may just be in that transitional phase where LO is about to shorten her naps on her own, but in the meantime it's making bedtime a disaster! That definitely was the case with us, and like I said -- i didn't realize he wasn't sleepy enough before bed, probably due to the naps.
You could try shortening the PM nap, like PP suggests, and see if that makes a difference. I think some of these bigger changes they go thru - like transitioning down to 2 naps from 3- cause some sleep disruptions, but it DOES get better. For awhile, I wondered if it would, but it definitely has.