Hi Matt ress,
You’re right that the density of latex varies with the ILD. Density also varies with the type and blend of the latex (Talalay is less dense than Dunlop and synthetic latex is less dense than 100% natural latex). If the type and blend was the same then they would be different densities.
Latexco is a distributor for many different types of latex from their own continuous pour Puralux (which is a blend of natural and synthetic latex) to molded Dunlop that is also a blend to molded Dunlop that is 100% natural.
Assuming that the 4 lb refers to the 24 ILD then it’s clearly not the 100% natural Dunlop (you can see the density/ILD comparisons here) and I would suspect that it’s the Puralux.
The Puralux can come either with holes or without holes. They are poured in layers up to 4" thick and then the holes are punched in afterwards if desired (the holes slightly lower the ILD). There is a hole punched version of Puralux that is 24 ILD that is about 4 lb density so this would make sense. If this is correct there is a 32 ILD version without holes that is about 5 lb density and a 40 ILD version which also has no holes is about 5.6 lb density.
Phoenix