@greeniiz111,
My niece, now 14, was apparently born with diabetes and has been insulin dependent since she was an infant. She understands her illness, gives herself her shots, regulates her glucose levels, and monitors her own diet and she has resolutely accepted all that goes with it since she was tiny. This is something that nobody would wish on any child, but to her it is no big deal and she has the world's best parents who were not overwhelmed by it and learned to deal with it and manage it until she could do that for herself.
Last year her twin brother was also diagnosed with diabetes. It has been easier for him because he already had dealt with his sister, but it was also more difficult for him because he didn't grow up with it himself and the disciplines required are tougher for him to learn and deal with. He's getting it done though.
At four years old your niece will also have some difficult in adjusting to a somewhat different routine and lifestyle and may be less able to understand than my nephew was, but she will adjust and despite the necessity to monitor her blood sugar, manage her diet, and probably deal with insulin injections, she will otherwise be a perfectly normal happy little girl.
Do give her parents extra moral support though. Dealing with the necessary disciplines of routine and monitoring along with understandable increased anxiety for awhile will be stressful for them. But millions of people live normal lives with this illness.