Can data be transfered from old phone to new phone already in use without losing its data?

sync your iphone 4 with icloud. i assume its at 7.1.2 and does not have icloud storage library. although, if it is ios 8.3+ you can upload your pictures there.

if not at ios 8.3 or later and really want those pictures, import them to a computer and then sync them back to the new phone.

as for the other content such as messages, you wont be able to restore those unless you restore the backup.

information such as contacts, calendars, reminders, keychain, notes, news and wallet are attached to your icloud. so as long as you sign in to your iphone 6 with the same icloud account on your iphone 4, that information will be mirrored. any changes done to either phone will reflect on the other

you can also try plugging your iphone 4 into a computer and then from the menu bar, File>devices> transfer purchases from "your iPhone name"

then sync that to the new one, in case you're missing one time only downloads such as books or ringtones

or you can also try syncing the new phones information to the computer, then erase all content and settings which wipes the phone clean, from there you will be in the setup menu where you can restore your icloud or itunes backup. make a new backup of your iphone 4 and then restore that to the 6. after you finish that. sync information back from itunes

The other way is to use iTunes - whether you are using a Mac or Windows, restore the old phone to iTunes and then restore the new one from the backup

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doing that will mirror the iphone 4 to the 6 but he's currently using both. this is a way of migrating his data to a new device. or syncing it to the other one and then backing the old iphone to restore to the new phone

What data or media, specifically, are you wanting to "transfer" to the new device? If it is media, was the media purchased under the same Apple ID as the one that is being used on the new device? Are the devices signed on to iCloud using the same iCloud ID?

There are several ways to sync data and media to multiple devices without affecting what is already on the device, but it is really dependent on the Apple/iCloud/iTunes IDs being used, the content of the iTunes Library, and whether or not iCloud is being used.

Can you provide more specific information about what it is you are wanting to transfer, and whether or not the same Apple/iCloud/iTunes Apple ID is being used on both devices?

Cheers,

GB