I did this upgrade recently.
Compared to previous major upgrades, it went very smoothly for me.
The official Fortigate doc on the upgrade path is here:
http://docs.fortinet.com/fgt/FortiOS-Upgradepath.pdf
Depending on your current 4.2.x firmware, you might need to go to 4.3.6 before going to 4.3.11 => 4.3.15
Here are a few relatively minor things I noticed after the upgrade:
1. The names of several applications changed or were removed. My application sensors had strange entries after upgrade and I had to edit the contents so they didn' t say things like, " Unknown Application."
2. I think I had one or two application sensors that had a space in the profile name that prevented it from displaying in the GUI. I had to rename it in the CLI. In general, you should insure there are no spaces in any profile names before doing the upgrade.
3. I had to update/upgrade the appropriate FSSO software on my servers to the latest version to prevent a few login bugs.
Bill
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Fortigate 600C 5.0.12, 111C 5.0.2
Logstash 1.4.1