Four lb. density foam weighs four pounds per cubic foot. High-density foam is higher quality than low-density foam because a higher density increases the foam's "memory" or ability to both support and conform. Foam densities under three lbs. are considered low density; any foam over five pounds is high density. Both three and 4-pound memory foam fall within the medium-density range, and it is certainly possible to buy a quality memory foam mattress in either of these densities.
One important difference between three lb. and four lb. memory foam is durability. Four lb. memory foam will simply last longer than three pound foam. When it comes to memory foam density, you get what you pay for; four lb. foam costs more, but holds up better over the years.
Higher density foams relieve more pressure than lower density foams, because they have more "memory," that is, the mattress molds to the body shape. If you suffer from aches and pains, investing in a four lb. memory foam mattress, over a three lb., may be a worthwhile investment.
Softness vs. Firmness
Three lb. memory foam does have a possible advantage over four lb. foam: it's softer. This difference exists regardless of the temperature of the foam; though memory foam softens when it is warm, higher density foam softens less when warm than low-density foam. Soft memory foam allows you to sink into it like a cloud, so weigh support against comfort when you decide.
Density is the main determinant of price when comparing memory foam products of the same size. Though both three lb. and four lb. are medium-density and good quality foam, four lb. foam is more expensive, because of its longevity compared to three lb. foam.