Would installing Origin and TS4 affect my Sims 3 install?

Origin will notice your TS3 install and ask if you want to update it (meaning Patch 1.69). You can refuse, thus keeping the disc install intact.

There have been reports of players agreeing to a TS4 patch further down the road and Origin then also patching TS3 anyway, but they can see it happening if they watch and abort it. Haven't seen these reports for a very long time though, so wouldn't know if this still happens but that couldn't have been intended.

Not sure that separate Origin/EA accounts would help. Seems like separate Windows user accounts might, though again if things were to work as intended that shouldn't be necessary either.

*raises hand* I use Origin for other games including S4 and still manage to keep my S3 install on patch 1.67. :)

Keep automatic updates turned off for the Origin Client AND games. I don't let anything update on my computer without my say so. :D
Like igazor says... When you update something on Origin, just keep an eye on it making sure it doesn't get carried away. I've never had that happen personally but I still keep watch. I go into offline mode if I don't need to be online.

When you have the Origin client installed... at the top Application Settings > under Client Update
turn off Automatic game updates | Automaticlly update Origin | Participate in Origin client betas

Thank you both! It's good to hear that it should be alright if I keep an eye on things. :)

I've downloaded Origin and when I was installing it, there was a tick box to automatically keep Origin and games up to date, so I unticked that and afterwards checked those settings SimplyJen mentioned and they were already greyed out, so hopefully it should be safe from the update now!

I'm currently downloading TS4 and I can see the 'update available' on TS3 which obviously I'll keep away from lol :D I think it's gonna take a while to download TS4 but I'll definitely keep watching while it's going on and then switch to offline mode once everything is finished.

Thanks again to both of you for your help! :)