With this:

    E = Struct.new(:name, :size, :type)
    def ls = Dir.children('.').map{ s=File::Stat.new(_1); E.new(_1, s.size, s.file? ? 'file' : 'dir') }

This becomes valid Ruby:

    ls.find_all{_1.type == 'file'}.sort_by(&:size).take(4).map{ {n: _1.name, s: _1.size } }.each { puts JSON.pretty_generate(_1) }

(drops your size formatting, so not strictly equivalent)

Which isn't meant to "compete" - nushell looks nice -, but to show that the lower-threshold option for those of us who don't want to switch shells is to throw together a few helpers in a language... (you can get much closer to your example with another helper or two and a few more "evil" abuses of Ruby's darker corners, but I'm not sure it'd be worth it; I might a wrapper for the above in my bin/ though)