you don't stack the catches, you only take the highest catch, so it'd be +4 or +5, for a total cost of -4 or 3 depending on how you Adjudicate how easy it is to get.
I might be nice and say it only cost 4 refresh because, if the ghost PC wants to do anything useful, they'll also need to take Poltergeist. You are looking at 9 refresh in powers.
And welcome to the boards.
Edit: when they talk about stacked catch they mean when you stack immunity and toughness
Like, you might have mythic toughness to every thing but cold (+3 catch)
And physical immunity against fire (bypassed by everything except fire) +5 catch
My numbers my not be correct but it's just an example.