As the owner of an Indian-based web and mobile development firm that has recently been going through the sale process, as well as an American cloud-based company we sold, I know what information you are looking for.
It's great that you are planning ahead 3-4 years. That's a great start. The best way to maximize the valuation of your company during the sale is to generate recurring revenue leading to sustainable profits in the 20%+ range. You should examine ways to generate recurring revenue from your clients and not depend solely on their returning for a rebuild years after the initial engagement. Consider a support package, or some value-added services on a subscription basis such as SEO, technical support, server management, etc. If your firm doesn't have those skills, you should consider partnering with another firm who can offer them under your brand. I suggest finding an offshore firm with a long track record and a commitment to service quality whose services you can mark up an additional 50% or so and resell. These relationships become critical to the sale process as well, so it's important to get started early and find the right partner.
There are no default ways to value your company, though there are a few accepted valuation methods. It will be largely up to you, as the business seller, to make and negotiate your position and shop around for different buyers. What I can tell you for certain is that you will be in a weak position if you don't get recurring revenue in place. Even moderate recurring revenue can generate 2x returns on your valuation. You should have at least 2 years of solid growth in these revenue channels before you approach buyers.
If you want, we can schedule a quick 15-30 minute call to lay out a strategy for getting the right price for you business.