Early Lockdown??

I would recommend against that. I just did something similar and it made for an exhausting long hatch with too much possibility of screwing up good chicks (having to reach in to get chicks while other eggs still aren't hatched). I had some early hatchers and some late (I set several different breeds). It turned into a 3-4 day hatch with a really disgusting incubator afterwards.  Yuck. I certainly won't set that way again.

I think you should lockdown day 18 and make sure you have a divider between 1st hatch and 2nd because your going to have chicks hatching when the 2nd group are supposed to go into lockdown and you don't want the chicks disturbing those eggs.

Four days apart isn't too awkward. I've done staggered hatches before that have been much more complicated, and still had a reasonably good hatch rate. If I were you, I'd monitor humidity/moisture loss as carefully as possible in order to make sure that neither batch of eggs is losing too much or too little moisture, and hold off on lockdown with the first batch of eggs as long as possible. Maybe stop turning them on day 18 just like normal, but wait till day 20 to bump up the humidity. If your incubation humidity has been okay up till then, raising it a couple of days late for the first eggs and a couple of days early for the second eggs shouldn't do any harm at all. When you bump up the humidity on day 20 for the first eggs, you can stop turning the second set and go into proper lockdown with all of them.

Opening the bator to take out the first lot of chicks while the second lot are still hatching poses a slight risk of shrinkwrapping some of them, but as long as you keep your humidity up high and get in and out as quickly as possible, you shouldn't have too many problems. Personally, I don't worry about shrinkwrapping chicks by doing this, as in many staggered hatches and many openings and closings of the bator door mid-hatch, I've yet to shrinkwrap a chick. That's not to say you'd be as lucky, just that it's maybe not as big of a risk as most people think it is. Good luck with your hatch!

Have you ever used the LG before? Did your friend give it to you because they didn't like it? The LG is infamous for being difficult to get along with.