This will be my last post. So, feel free to add what you like. However, since the OP wants to mainly play "arcade games" (his words) these games (like all games) do not JUST rely on the GPU, but on the CPU and RAM, too. Getting more system RAM will SPEED UP HIS ENTIRE SYSTEM, allowing for more data to be stored in the ultra-fast RAM so the CPU is not waiting on data to process, etc. Getting more system RAM would also provide more RAM to be used by the OP's on-board GPU, too. And since adding a few sticks of RAM would most likely be less expensive than a dedicated video card, system RAM would be the least expensive way to speed up both his games AND his entire system at the same time.
Getting a new video card would definitely help the OP's gaming experience on their PC. However, the low amount of RAM would still potentially throttle games as data crunching is happening, too (no matter how well Windows 10 now scales). More system RAM would improve the overall performance of the entire system, including the on-board GPU. So, for a lesser price, the OP can have a better running PC overall AND a better gaming experience.
Heck, if possible, the OP should do both. There is no reason to run only 4 GB of system RAM. None ... unless the motherboard only supports 4GB ...