whether you go to 600 or whatever, your in a 4x4 tent so heat will be the issue. My self I like having two diff light sources. that way you have a better light cover with less shadowing, maybe use the 400 hps and get a 400 mh. duel spectrum baby, and you can flower with mh. or get a 600 watt digital hps/mh that's has wattage adjustment. you benefit from both. And as far as yield goes it all has to do with Strain type, nutrients, is everything dialed in optimum. light spectrum. grower? you can have two exact set ups with two diff growers and you will get different results.
The Rev (Skunk magazine Cultivating editor) used to only flower with MH. mow he does both MH and 400 watt led's using his TLO method.