Most people would probably suggest that it is time to get a new(ish) computer, but I am a fan of utilizing what you have a if you can achieve the performance you are looking for.
What do you plan on doing with your Mac, like apps and such?
I wanted to upgrade it with an SSD 500 GB
You will see a huge performance increase just by switching to a SDD. Not only will things respond a lot quicker, but if you are page swapping a lot since the 4GB of RAM then everything would just seem slowed down with a HDD.
The question would be whether to do an internal upgrade or external. I would suggest doing the internal upgrade for the best performance. It isn't hard to do it yourself, but it could intimidate some people.
With replacing the internal HDD with a SATA3 SSD, you could see potential speeds up to 750MBps, but real world about 450-550MBps. Very fast, and a huge improvement over your aging HDD.
If using an external SSD, you have a few options: USB2, FW800, TB1, and a SD card slot.
Ideally, the TB drive would be the best, but probably the most pricey. I have found some cheap TB1 and TB2 drives on eBay with HDDs and I just swapped them for SSDs. LaCie Rugged Thunderbolt drive can be found on eBay for pretty cheap and swapping the HDD for a SSD can be done in less than 60 seconds. I have a few of these and get about 350-450 MBps speeds on the LaCie Rugged Thunderbolt drives with a swapped SSD.
While I never have done it, I have read that you could use NVMe TB3 drives with the Apple TB3>TB2 cable, giving you the highest potential speeds of over 1GBps and you could use that drive with a future Mac, but that would probably be a little on the more pricey side since the cable alone is $50.
With a FW800 drive, you would probably see a decent noticeable snappiness to your OS, but speeds would be capped at 100MBps, probably still better than your old HDD though.
The Mid-2011 iMacs does not have USB3 so you would be stuck at 60MBps speeds with an external USB2 drive.
I have never tried it, but the SD card slot is capable of speeds up to 625MBps, but AFAIK the fastest SD cards are 300MBps. Still a huge performance increase over the HDD, but I never actually used a SD card for an OS. I know it works, but I never actually tried it. It is on my list of things to try soon. Even if it works great, the fast SD cards are still pretty expensive when compared to SATA SSDs, so most of the other options listed above would be cheaper per GB.
You can find 2x4GB sticks for less than $30 giving you a total of 12GB of RAM. Depending on what you do with your Mac, you may never page swap again with 12GB of RAM.
