Shall I add a RAM or change the PC?

I have a Lenovo laptopi5with 4GB RAM. I used to browse the net and do some light work on this.
It hangs after continuous use of 1-2 hours. The keyboard also becomes hot.
I use good antivirus software and update it regularly.

Could you please advice on boosting its performance?
Thanks

do a fresh windows install if you are running windows. Make sure you are letting it breate. The vents are typically on the bottom and if they are blocked aznd it can get fresh air it will get hot.

Is the fan working? The blow out the hot air from the cpu?

4gb of ram is plenty for web browsing and typically some work depending on what work is for you and what programs etc

These days chrome is hogging up 1GB depending how many tabs you got open, Windows 10 uses up about 512mb while doing nothing.
4GB more won't do much harm, but it's not really worth spending much on a laptop anyway.

If it's over heating, it will cause a bottleneck with the CPU, take the back off, clean out the dust, you could try getting some good thermal paste and re-applying. Just sounds like a heat issue bottlenecking the CPU.

I always use pressured air cans trough my laptop, i bought it in 2015 and works great still, maybe once in ever 6 months I clean it, taking it apart and spray some canned air in it and putting it back together. (Lenovo T450s) easy to take apart and reassemble.

There's absolutely no need to change pc, its not receiving enough air...clean the dust inside

Todays is better to use 16GB or 8GB but the RAM was not cheapest than 1 year ago