Are there any prophylactics (e.g., supplements, special diet, otc or other pharmaceuticals) that you'd recommend for someone heterozygous apoe4?

Ignore it: No magic pills or food will modify your risk. Truth is, life's too short and difficult already without fretting about your having a common genetic variant that puts you at slightly increased risk for disease in old age. I have not seen strong evidence that supplements prevent alzheimer's. Take the same good care of their arteries / lipids as others do. Enjoy life.

Yes: Although the proof is not definitive, recent studies show a strong correlation between being fit, eating berries and wild fish like salmon rich in omega-3s, and learning new skills. One study last year showed that people who exercise and use their brains at the same time like on some special bicycles do very well.