Need advice ; therapy

I have 2 little. My oldest is 8 and my youngest is 4. My 8 yo has autism and ODD. He has explosive meltdowns that normally consist of yelling, screaming, aggression, etc. He’s never really aggressive towards my 4 yo minus 2 times in which crisis team was called for him. I’m actively searching for a residential facility for him until my insurance can finally approve his autism therapy but until then we’re chugging through it. He also has a psychiatrist and on medications. My 4 yo however has always been really chill and laidback and sweet. However in the past 3-4ish months, she’s been very reactive to his behavior so much to the point that if even my 8 yo tries to climb in bed with us, my 4 yo will start screaming and trying to slap him telling him to go away. It’s a constant fight between these 2. So my question or advice I’m looking for is, at this point should I seek therapy for my 4 yo also? I’m not even sure if therapists start this young because of how their brains are still constantly changing but it seems like all her reactions are all reactive behavior. I’ve taught both my kids the phrase “your feelings are okay to have, but your actions are not always okay to do” and it really isn’t doing anything.