can I increase RAM for iMAC 21.5/2.7QC/2x4GB/1TB ?

With great risk and difficulty. 2012-2013 21.5" iMacs can have the RAM upgraded by trained technicians, but the RAM is not considered user serviceable (complete disassembly/reassembly required).

I wouldn't recommend making the attempt. Slow performance can usually be more easily and safely fixed by reinstalling MacOS onto an external USB 3.0 SSD (or alternatively going ahead and buying a newer more powerful computer.)

To add to the excellent replies you already received the short answer is "no" but more RAM might not help anyway.

Fortunately it is very easy to determine if your Mac's performance is being limited by available memory. Please read Check if your Mac needs more RAM in Activity Monitor - Apple Support. If the "Memory Pressure" graph is frequently "red" your Mac would benefit from additional memory. If not, it won't. It's that simple.

8 GB should be sufficient for most purposes, although we have no way to know what those purposes may be in your case.

If your Mac isn't performing as well as you think it should EtreCheck is likely to provide some actionable information.

+1 on the external hard drive boot option. 95% of "slow iMac" reports here for 21.5-inch models introduced between 2012 and 2019 are due to the slow and under-spec hard drives Apple used in entry-level units. If that is the case, adding all the RAM in the county won't change that.

However, we would be remiss to assume your Mac does not have a faster storage option installed, like a factory solid-state drive (SSD) or Apple's incarnation of the hybrid drive, Fusion. Some slow issues with ether can be addressed at home without opening a computer the Apple did not design to be opened.

If you wish a data-driven evaluation of your computer's hardware , software, and performance status that can be done in this text-only setting, you can post a system configuration snapshot. Fortunately there is a safe, secure way to do that without our playing a protracted game of "20 Questions" with you that could go on for days.

We can quickly and within the confines of these forums help you determine what issues are at play if you use EtreCheck Pro, available here:

https://etrecheck.com/index

The free version will do nicely for this purpose, although the app is worthy of our financial support.

We can see hard data about drive performance, software issues, and RAM usage. Etrecheck is the development of a long-serving and trusted contributor here expressly for displaying information in these forums to help us help you. By design it will not reveal any personal or secure information.

See this excellent User Tip on successfully posting long text reports like EtreCheck.

How to use the Add Text Feature When Post… - Apple Community