4.2.1 is the app version. FW version is listed in settings about. that said if you have 4.2.1 app you should have the latest FW.
Can the photos be deleted from the native iOS Photos app?
How much free space is on the iphone internal storage?
Are any of the photos live photos, slow motion videos or edited in any other way?
If you try deleting them in smaller chunks say 10 photos does it complete without error?
