why CS GO Lags on i5 4th gen and 16gb ram?

It's something that is VERY hard to answer since it can be a stupid amount of things from your install to a driver incompatibility.

So try to find out: is there anything that you can remove that when you don't have that on you do not lag? Are your drivers updated? Is there an effect in particular that your graphics card just sucks at?

Examples:

• Recently I had problems with Apex Legends. Just seemingly random spurts of lag. But I was running Apex Tracker in the background. Desactivating it stopped the lag.
• On one of my old computers, I had a shitty graphics cards. After a little while, I realised that any dynamic shadows would kill my performance. So every game, I would need to go to graphics options and put shadows to none or the lowest setting. Everything else could stay medium/high, but if there was shadows, blam. lag.

For your problem in particular, I would go for a driver problem. You have a setup that if I read right, you should blow any requirements of CSGO out of the water. So something is misfiring.

But you might be in luck: CSGO has a huge community and tools to see exactly how your computer is trying to run it so search around, you might find a tool that can search and benchmark your PC to find what problems it encounters.