Hello W 4 days till you get the light. Well you have no choice really all I can think of doing is putting them near a window and be prepared for them streching, so you might have to put some sticks in them to help support them.

I'm just worried about the length of day light they will get this time of year, it's going to be less then 12 hours, what effect it will have on them at this age I don't really know ?
You're just going to have to try it for now and as soon as you get the HPS get them under straight away and run on a 18/6 light cycle. An MH would be much better for the vegitive growing (18/6), but sometimes you just have to work with what you have
If the ballast for the HPS can also take a MH bulb then it would be better to get one of those.

Just give it a try W with what you have and see how it turns out, when you do the next grow you will have learned a lot from this one


good luck ...............mand xxx