It is not possible to install the NVIDIA K4100M GPU in an iMac, as this GPU is designed for use in high-end mobile workstations and is not compatible with iMac systems.
Additionally, even if you were able to find a compatible GPU upgrade for your iMac, it is important to note that upgrading the GPU in an iMac is not a simple or straightforward process. The iMac is a tightly integrated system with custom components, and any upgrades or modifications should only be attempted by experienced technicians with the proper tools and expertise.
In terms of upgrading to an SSD, this is a more straightforward process that can significantly improve the performance of your iMac. However, it is still important to take the necessary precautions and follow the proper procedures to avoid damaging your system or losing any data.
Overall, while it may be tempting to upgrade the GPU in your iMac to improve performance, it is not a recommended or feasible option. Instead, focus on upgrading other components such as the storage and RAM to improve performance and extend the lifespan of your system.