Is the phone's lock screen PIN secure enough to be in the repair shop for a few days?
I'd say no. There are forensic-type applications can that extract your phone data without requiring the screen to be unlocked. It's fairly easy to backup and then wipe your phone, and later restore it from that backup. Might as well do it and rest comfortably knowing none of your personal data is in the hands of someone else.
Besides, you may not get your exact phone back. They may be able to swap it for a new phone or a refurbished phone if they're an Apple Authorized repair depot. In that case your data may be left on your old phone and sent off to a depot for reconditioning.
Who would want that uncertainty?
Backing Up Your Phone
Before you wipe your phone to send it in make sure you have a backup!
If you're using iCloud back up your phone go to Settings -> iCloud -> Storage & Backup and click the Back Up Now button to do a backup.
If you're using iTunes to backup your phone connect to iTunes and do a backup.
Wiping Your Phone
DOUBLE CHECK YOU'VE MADE A RECENT BACKUP!!!
To wipe the phone go to: Settings -> General -> Reset and click the Erase All Content and Settings button. This will set you back to factory defaults.
Send it in for repair.
Restoring Your Phone
When you get it back it should behave as if it's a new, never used, iPhone. In this case, after you pick your language, you'll be asked if you want to set it up as a new phone, restore it from an iCloud backup or restore it from an iTunes backup.
Select Restore from iCloud Backup and follow the prompts if you used iCloud to back up your phone.
Otherwise select Restore from iTunes Backup and follow the prompts.