Thread scheduling is a core functionality of operating systems, optimizing CPU resource utilization through dedicated algorithms. However, modifying kernel-level scheduling in Linux is complex and error-prone. User-space scheduling offers a promising alternative, yet existing implementations struggle with speed, flexibility, and robustness. This paper introduces F4, a user-space thread scheduling framework that shifts scheduling algorithm deployment from the kernel to user space. By allowing the kernel to delegate scheduling decisions through interfaces like system calls and eBPF hooks, F4 enables user space to develop and implement its own scheduling strategies. This separation of scheduling mechanisms and policy definitions enhances efficiency and adaptability. F4 improves communication through message queues and the shoot system, accelerates execution with inter-processor interrupts (IPI), and increases interference resistance through dynamic priority adjustments. These innovations allow F4 to achieve fast, flexible, and stable-fortified thread scheduling in user space, addressing the limitations of current user-space scheduling solutions.