abc="[abc [d1 e] [f2 [g h3] i4]]"

def split_1 (ln):
    if ln.startswith('['):
        ln=ln.lstrip('['); ln=ln.rstrip(']')
    ln=ln.replace('[',' [')
    ln=ln.replace(']','] ')
    ws=ln.split(); ws.reverse()
    os,n,ns = [],0,[]
    while ws :
        w = ws.pop()
        if '[' in w: n+=1
        if n : ns.append(w);
        else : os.append(w)
        if ']' in w:
            n-=1
            if n==0: os.append(' '.join(ns)); ns=[]
    if len(ns): os.append(' '.join(ns))
    return os

Parentheses are smooth and curved ( ), brackets are square [ ], and braces are curly { }.