To handle a scenario with 4 Internet lines of 10 Mbps each and support up to 400 users simultaneously, it is crucial to select a MikroTik equipment that can not only handle the load balancing workload, but also has sufficient processing capacity and memory to manage the traffic of all users efficiently.

Factors to consider include:

• Processing capacity (CPU): You need a router with a powerful processor to handle multiple connections and tasks simultaneously without degrading performance.
• RAM: Essential for handling large routing tables and multiple connections.
• Number and type of Ethernet ports: You must have at least 4 Ethernet ports for Internet lines, in addition to the ports necessary for the LAN network.
• Support for load balancing: The ability of the device to efficiently support load balancing between multiple WAN connections.

Given this, a model that could be recommended is the MikroTik Cloud Core Router (CCR). In particular, the CCR1009-7G-1C-1S+ It is an excellent option that meets these requirements. Here are some details about this model:

• CPU: It has a powerful 9-core Tilera CPU that can handle the intensive data traffic needed for 400 users and the load balancing of 4 Internet lines.
• RAM: It comes with 2 GB of RAM, more than enough to handle multiple connections and routes.
• Ethernet ports: It has 7 Gigabit Ethernet ports to connect your Internet lines and your LAN network. It also includes a combo port that can be used as an Ethernet or SFP port, providing flexibility for fiber optic connections.
• SFP Support: Additionally, it offers an SFP+ port for 10Gbps connections, which is useful for high-speed uplinks or network storage connections.
• Performance: This model is capable of handling packet routing at high speeds, which is essential for maintaining a smooth user experience.

This router is suitable for businesses that need high performance and stability in routing and load balancing. Its ability to handle multiple WAN connections makes it ideal for your situation, providing a robust solution to manage the Internet traffic of 400 users.

You can also use an RB3011, RB4011 or RB1100AHx4 unit.

However, it is important to also consider aspects such as budget, available physical space, and whether you need specific additional features such as VPN, traffic filtering, or advanced quality of service (QoS).

It is also advisable to review the latest technical specifications and any product updates directly from MikroTik or specialized consultants to ensure that the equipment meets your specific needs.