Personally, I stop feeding milk when their tummies start to round out. You can overfill the abomasum, and they will get digestive upsets if the milk overflows it.
What type of lambs are they? 4x 300ml at 15 days old is quite a lot of volume. I think Lamlac suggests 1.5L per lamb per day, does it? The milk I use is a 1L per lamb per day per lamb, but I rarely feed that much, and mine are good commercial Texel type lambs. At 15 days, mine would be on 3x 250ml, and I'd be working them towards 300ml for the first and last feeds of the day.
If their tummies are very full after each feed, and they're on 4 feeds, they probably aren't getting hungry in between milk meals. So then they don't eat much straw, or hay if offered, which they need at that age.