Id go through the whole route with 2 x 1000w
600w aint enough for a 4 x 4. 1000w Hortilux is made specifically for a 4 x 4. If ya got 2 x 4 x4 which equals 4 x 8 or 32sq/ft you need 2000w/HID for the space.
I start seedlings-flowering under 1000w Hortilux and have used 1000w HID for 40 years. Since 1978 which I used Halide then, as none of the HPS were Blue enhanced.
But I was ging to do what you suggest, Id give them 1000w First, and then finish with 600w.
You aren't thinking of using a 600w for the whole 4 x 8??? Useless for that.