Q: I have a 4 year old mac book pro 13 inch with Mac OS X version 10.6.8. should I upgrade and to what?

If you don't need to, then don't. If you want to upgrade, then:

Upgrading to Yosemite

You can upgrade to Yosemite from Lion or directly from Snow Leopard. Yosemite can be downloaded from the Mac App Store for FREE.

Upgrading to Yosemite

To upgrade to Yosemite you must have Snow Leopard 10.6.8 or Lion installed. Download Yosemite from the App Store. Sign in using your Apple ID. Yosemite is free. The file is quite large, over 5 GBs, so allow some time to download. It would be preferable to use Ethernet because it is nearly four times faster than wireless.

    OS X Mavericks/Yosemite- System Requirements

      Macs that can be upgraded to OS X Yosemite

         1. iMac (Mid 2007 or newer) - Model Identifier 7,1 or later

         2. MacBook (Late 2008 Aluminum, or Early 2009 or newer) - Model Identifier 5,1 or later

         3. MacBook Pro (Mid/Late 2007 or newer) - Model Identifier 3,1 or later

         4. MacBook Air (Late 2008 or newer) - Model Identifier 2,1 or later

         5. Mac mini (Early 2009 or newer) - Model Identifier 3,1 or later

         6. Mac Pro (Early 2008 or newer) - Model Identifier 3,1 or later

         7. Xserve (Early 2009) - Model Identifier 3,1 or later

To find the model identifier open System Profiler in the Utilities folder. It's displayed in the panel on the right.

     Are my applications compatible?

         See App Compatibility Table - RoaringApps.

There is always a risk when upgrading. Here's how you ought to proceed:

A. Repair the HD and permissions in that order.

Repair the Hard Drive and Permissions

 

Boot from your Snow Leopard Installer disc. After the installer loads select your language and click on the Continue button. When the menu bar appears select Disk Utility from the Utilities menu. After DU loads select your hard drive entry (mfgr.'s ID and drive size) from the the left side list.  In the DU status area you will see an entry for the S.M.A.R.T. status of the hard drive.  If it does not say "Verified" then the hard drive is failing or failed. (SMART status is not reported on external Firewire or USB drives.) If the drive is "Verified" then select your OS X volume from the list on the left (sub-entry below the drive entry,) click on the First Aid tab, then click on the Repair Disk button. If DU reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported click on the Repair Permissions button. Wait until the operation completes, then quit DU and return to the installer.

 

If DU reports errors it cannot fix, then you will need Disk Warrior and/or Tech Tool Pro to repair the drive. If you don't have either of them or if neither of them can fix the drive, then you will need to reformat the drive and reinstall OS X.

B. Create a bootable clone of your system using the Restore option of Disk Utility.

Clone using Restore Option of Disk Utility

 

  1. Open Disk Utility in the Utilities folder.

  2. Select the destination volume from the left side list.

  3. Click on the Restore tab in the DU main window.

  4. Select the destination volume from the left side list and drag

       it to the Destination entry field.

  5. Select the source volume from the left side list and drag it to

      the Source entry field.

  6. Double-check you got it right, then click on the Restore button.

Destination means the external backup drive. Source means the internal startup drive.

C. Verify you meed the requirements to install Yosemite.

      Macs that can be upgraded to OS X Yosemite

         1. iMac (Mid 2007 or newer) - Model Identifier 7,1 or later

         2. MacBook (Late 2008 Aluminum, or Early 2009 or newer) - Model Identifier 5,1 or later

         3. MacBook Pro (Mid/Late 2007 or newer) - Model Identifier 3,1 or later

         4. MacBook Air (Late 2008 or newer) - Model Identifier 2,1 or later

         5. Mac mini (Early 2009 or newer) - Model Identifier 3,1 or later

         6. Mac Pro (Early 2008 or newer) - Model Identifier 3,1 or later

         7. Xserve (Early 2009) - Model Identifier 3,1 or later

     To find the model identifier open System Profiler in the Utilities folder. It's displayed in the panel on the right.

D. Download the upgrade installer for Yosemite from the App Store.

E. Install Yosemite.

I do not know what you mean about risk. You should always have a backup incase a problem occurs during the update.

I would clone the existing disk to an external disk by;

http://nyacomputing.com/how-to-create-a-bootable-clone-of-your-mac-hard-drive/

Some apps may not work, Kappy mentioned that by:

Are my applications compatible?

         See App Compatibility Table - RoaringApps.

How much memory do yo have. Apple says that 2 GB is the min but even with 4 GB it is laggy and at least 8 GB is recommended.