Hi
Can someone tell me the differense between the two app`s which one to use. I have iPad A1893 and Phantom4.

If you are developing new applications to use with DJI drones, you could use the SDK (software development kit).
If you are wanting to fly, you need DJI Go 4.

Hi there, thank you for reaching out and we really appreciate you for being interested in using DJI Product.  With regards to that Captain @Labroides correct about it. If you are into developing you may take advantage the SDK and if ever for recreational use or just a normal set up you need to use the default app which is the DJI Go 4. Thank you for your support.