The four major online marketing domains are search engine optimization (SEO), social media marketing, email marketing, and content marketing. SEO involves optimizing your website to rank higher in search engine results pages (SERPs) for specific keywords. Social media marketing involves promoting your brand and engaging with your audience on social media platforms like Facebook, Twitter, and Instagram. Email marketing involves sending targeted emails to your subscribers to promote your products or services. Lastly, content marketing involves creating valuable and informative content to attract and engage your target audience. By utilizing these four domains, you can create a well-rounded online marketing strategy that will help you reach and engage with your target audience effectively.