It is a little complicated. ApoE transports cholesterol. The different isoforms have different abilities to bind cholesterol. ApoE2 is best, then ApoE3 with ApoE4 the worse but it still has some ability to bind and transport cholesterol. When cholesterol levels are low on the blood, ApoE4 might do an ok job but when cholesterol levels are higher, it can’t keep up. Also consider that there are heterozygotes (ApoE3/ApoE4) that fair better and homozygotes (ApoE4/ApoE4) that have the highest risk.