No personal experience from me Emma, F wouldn't even entertain the idea until after 6mos even if I did want to start early!!
Here are my thoughts. I personally feel 4mos
may be too early for a lot of babies. But the only ones who know this are the babies and mommies. If you think she seems interested, you can always give it a try to test it out. Just make sure you don't push the issue if it's a no-go. I know some babies go nuts for solids around 4mos, but others are just not ready, and I hate it when moms feel pressured to do it early just because it worked for other moms and babies.
I'm assuming too you mean doing the puree route. If BLW is something you are interested, D really has to be 6mos to be developmentally ready. She needs to be sitting upright on her own. I really think though that any baby starting solids needs to be sitting on their own no matter what for safety reasons. Eating while reclined is not safe for anyone!
(((hugs)))