Is there data or memory loss when taking phone apart?

Generally no, there are a couple of circumstances that I have observed when working on iPhone 4 or 4s models.

1. When you disconnect the battery the time and date will reset to 8:00am 1/1/1970 (don't ask me why 1970, I have no idea). This usually is changed back to current time by default when your phone gets signal after repair. Depending on what software version you are on, sometimes the signal doesn't connect BECAUSE of this time/date mismatch with the server. In this instance either set the time and date manually or connect the device to a wifi network (it then should do it automatically within about 30 seconds). After you reset the time and date, be sure to turn AIRPLANE mode on and then off again to reset the network.

2. In very rare circumstances the library file for the camera roll can become corrupted and result in your pictures not showing up on the phone (don't worry they are still there) but appear when you connect the device to a pc and explore the DCIM folder. This can be repaired but I can't really explain it here, Just google "iphone corrupt camera library" for instructions on how to fix this.

So long as you don't knock any resistors/capacitors/connections off the main board when repairing you should be fine. If you are concerned about any of your data ever, it's a good idea to backup your phone before repair.

Goodluck in your repair endeavors :)