Are they nursing?
Bf lambs at that weight would likely not survive here. But do know that fall born lambs do not typically weigh as much as winter born.
That being said,that weight is common for my hair cross ewes bred to hair rams or even Hamp rams. Lots of 5 to 6 lb trips and quads weigh a bit less. Most twins born out of 13 to 14 month old hair ewes were in that 4.5 to 5 lb range.