Our Free plan is limited to 3 social channels with some limited publishing tools. To work with 4 or more channels, you'll need to subscribe to a paid plan, which then includes all the advanced publishing, analytics and engagement tools.
On our paid plans, you can add as many social channels as you need since we charge on a per-channel basis. If you upgrade from the Free plan, you'd pay per social channel that you have connected, including the 3 that you already have connected. Once you upgrade, our advanced tools are unlocked for all channels.
Our current pricing can be found at https://buffer.com/pricing
We have a chart to break down which features come with each plan here.