@leeowensoas some can, others by the seat of their pants, most can't. Experience at pushing the envelope is a big factor. I don't solo +4/8 except specific enemy types and particular missions/arcs.
Of those that can +4/8 level 50 content, fewer are doing lower level content that way. That said there are some truly exceptional players.
Can you? Absolutely with the right build and practice, yes.