Need help figuring out how to save 1500 pictures between 2 iPhones and and iPad while traveling
For part of your solution you could try Dropbox to upload the files for later retrieval.

Download the app on you iPhone 4 and setup an account.

To upload your images follow the instructions here (with visual aids). Which are:


First navigate to the location within your Dropbox where you'd like
to put the photos. Then do the following:
Tap the ellipsis (...) icon at the top right of the screen to get a
drop-down menu.
Tap Upload (+) from the menu.


(Optional: To put the photos in a new folder, tap Create Folder from the menu first before uploading.)


Tap the ellipsis icon to get a drop-down menu.
Select the album you'd like to upload photos from.
Select the photos and videos you want to upload.
Tap the Upload button at the top of the screen.




You could install the dropbox app on your personal iPhone login and the download the images from dropbox. But oddly dropbox do not do a multiple download to the iPhone photo library only single. ( pain)

It is possible to use third party app and logging into your dropbox account through it would allow you to do multiple downloads



Also dropbox have a new auto upload which may help you when you take new photos.


  The Dropbox mobile app gives you different ways to upload files from
  your phone or tablet to your Dropbox account. Once files are uploaded,
  they will sync with Dropbox on your computer and all your other
  devices.
  
  Upload photos and videos automatically
  
  Dropbox can automatically upload photos and videos taken from your
  iPhone. This feature is called Camera Upload, and you can turn it on
  when you first install the app or by going to the app's settings. You
  can also choose whether to allow uploading when your device is
  connected to a cellular network.




Keep in mind you get about 2GB+  free.

1500 images may that up most of that space. You can see how much by looking at the usage settings on iPhone ( amount will include video sizes)