Q: iMac 10.8.5 OS 4GB memory late 2009 edition upgrade to Yosemite help

What is the exact error message? I'm guessing that it is telling you that your hard drive is too full. If that is the situation, you will need to delete files from your hard drive to make enough free space.

Yosemite only requires 2 GB of RAM. It also requires 8 GB of free space on the hard drive.

What is the exact error message?

Darren?

No problem :-)

I'm not sure what you are asking exactly.

• 21.5" iMac late 2009 - max RAM is 16 GB
• 27" iMac late 2009 - max RAM is 32 GB

You need to post the exact error message you are getting, as well as confirming what model of iMac it is. As Duane points out a 2009 iMac is not maxed out at 4GB.  

iMac (21.5-inch, Late 2009) - Technical Specifications

iMac (27-inch, Late 2009) - Technical Specifications

You need to find out exactly what you have and how much RAM it actually has currently.   You can do so from the Apple Menu->About This Mac option.

Without that, it's impossible to move forward.

OS X 10.10 Yosemite is no longer available for download.

If your Mac is a 2009 model running OS X 10.8.5 Mountain Lion, then you only other choice for OS X upgrade is to download and install the free OS X 10.11 El Capitán.

Newer versions of OS X need more than the minimum 4 GBs RAM.

Depending on your 2009 model iMac, you can install more RAM into that iMac up to, at least, 16 GBs of RAM.

Correct, compatible and reliable Mac RAM can ONLY be purchased from online RAM sources Crucial memory (http://www.crucial.com) or OWC. (http://www.macsales.com).

DO NOT PURCHASE RAM FROM LOCAL COMPUTER OR ELECTRONICS STORES!
Macs are picky about RAM and RAM from local sources will NOT work in a Mac.

Make a complete backup of your current system before downloading and installing OS X 10.11 El Capitán.

We need to the EXACT iMac model you have.

What Mac do you have?
To find out info about your system,
Click on the Apple symbol in the upper left of the OS X main menu bar. A drop down menu appears.
Click About this Mac. A smaller popup window appears. This gives you basic info like what version of OS X your iMac is running, the speed of your iMac's CPU and how much RAM is installed.
Click on the button that says More Info.
A larger window appears giving you a complete overview of your iMac's hardware specs.
Highlight all of this info and copy/paste all of this into another reply to this post, editing out your iMac's serial number before actually posting the reply.
This will tell us everything about your iMac so we may begin to help with your iMac issues.

This article shows each iMac (intel) and the slot configurations.

If the computer you have only has Two memory slots, then it is

not a Late 2009 build series. If you are counting unused slots,

then there may be two with chips and two without.

• Install memory in an iMac - Apple Support

{This article shows the earlier known supported memory totals;

later certain memory chip vendors had tested high quality RAM

and found these computer models in some cases can use more.}

• Identify the model of your iMac - Apple Support

See information sources such as http://mactracker.ca database

app for modern upgrade specifications for various Macs. Or

look into the http://everymac.com site to see specs for your iMac

and be sure to correctly identify the unit as there are several.

In any event, to get the total capacity supported in the computer

especially where four memory slots are involved, all four may need

to be upgraded to a new higher capacity RAM chip. The supported

total can be higher than the original Apple total, in some instances.

Good luck & happy computing!