In terms of what's needed to accomplish OpenGL 4.0 compliance in Mesa, the GL Shading Language (GLSL) changes is one of the big ticket items still left while the GL_ARB_gpu_shader5, GL_ARB_gpu_shader_fp64, GL_ARB_shader_subroutine, and GL_ARB_tessellation_shader are the other major OpenGL extensions to be implemented. Intel's driver is the best off with supporting the GL 4.0 work that's been completed while the state of the Radeon and Nouveau drivers vary in their GL4 extensions coverage -- Intel's Open-Source Technology Center spearheads most of the major GL advancements inside Mesa and are thus targeting their hardware driver while the Radeon/NVIDIA drivers lag behind to varying lengths in their adoption of new extensions.
For OpenGL 4.1 compliance, the GLSL changes are left along with the GL_ARB_shader_precision and GL_ARB_vertex_attrib_64bit while everything else is crossed off for at least the Intel DRI driver. With OpenGL 4.2 are more GLSL changes plus GL_ARB_shader_image_load_store. As soon as OpenGL 4.0 compliance is reached, it doesn't look like it will take long until hitting OpenGL 4.2 unless getting caught up by the many GLSL changes.
The OpenGL 4.3 and 4.4 support meanwhile is much further out with many of those work items remaining open. The full list of GL 4.x work to be done can be found via the Mesa Git documentation. Once OpenGL 4.x support is reached, the Mesa major version number will be bumped to Mesa 11.0 for signifying the support of a major OpenGL revision. Next month meanwhile The Khronos Group is expected to announce the next-generation OpenGL standard, which will be yet another large undertaking for open-source graphics driver developers to ultimately tackle for Mesa in due time.
Update: This morning Intel also took care of OpenGL 4.4's GL_ARB_clear_texture extension.