The 150 really is suited to max 3mm. tho my mount is 5mm and i get about 4mm penetration just (using argon/co2 mix), so full penetration on 4mm is a bit of a no no with that welder 
. So i'm planning a new miggy to get the job done 
Not the best person to offer advice but.....
Maybe slow down - 3-4mm is maybe on the limit of a 150 unless you V prep the parts



