This solution is specifically for those who haven't played, and have no intention of playing, Forza3.
As you may be aware, in order to get a Forza3 savefile to import into Forza4 you need to play the first race in Forza3, which will automatically pop an achievement. For those who do not wish to 'blemish' their gamercard with Forza3 (ruining completion percentage, for example) it can be a headache, but there is a way around it that involves racing offline.
So... for a step-by-step guide:
1. Disconnect from LIVE,
2. Play the first race of Forza3,
3. Create and save a vinyl group,
4. Back up to the dashboard,
5. Delete your profile from your Xbox (Profile ONLY, not including items),
6. Reconnect to LIVE,
7. Recover your profile from the cloud,
8. Go to Forza4,
9. Select import vinyl group,
10. Import Forza3 vinyl.
Doing the above has the effect of popping the opening Forza3 achievements offline, but by deleting your profile before reconnecting, your online card has no record of Forza3, leaving you with just the vinyl group to import.
Note: This solution was originally posted by me in the comments section of another solution, but I figured it was worth posting separately as the original author has not updated his solution to take account of the fact that it is possible to get this achievement without having Forza3 on your gamercard permanently.
Credit to Rykin Poe
for the knowledge that importing a vinyl group will pop this achievement.