What is the best herbal cream for a 4 month old baby with eczema?

Why herbal? Poison ivy is "herbal", but you wouldn't want it anywhere near your skin. "herbal" does not always equal "good", so you should not restrict your options to "herbal" products. If you're sure your baby has eczema (and not seborrhea or something else that looks like eczema), try 1% Hydrocortisone cream or lotion. If that doesn't work, you'll need to see a dermatologist.
Therapy for eczema. I use something called prean cream, an excellent barrier cream for dermatitis of all types. Herbal does not necessarily mean better. If this is persistent, please see your dermatologist.
One that skin likes. The key is moisturizing the skin, both inside and out. Keep baby well hydrated, do not let him get sweaty, use a mild soap without scents and try different creams to see what his skin likes. Apply moisturizers multiple times per day.