4 Kg is only about 5% of the weight of an average adult male. Now imagine that weight tied to different places on your body. On your back it might not be a big issue, but glued to the tip of one finger or maybe tied to one of your toes it could effectively disable or immobilize you.
The weight isn't the whole story, it's how it is placed that matters as well.
>The transmission technology will continue to be of great importance, as sensors weighing five grams are still too heavy for many animal species: 70 percent of bird species and 65 percent of mammal species, not to mention amphibians or insects, cannot be equipped with sensors using the current technology. The next generation of Icarus sensors will therefore weigh just one gram.
And the really bonkers thing about many birds is that the single gram will still be a sizeable fraction of their total body weight. Chickadees and nuthatches (and other birds of that size) weigh somewhere between 10 and 15 grams, usually. Zebra finches are about the same. It'll be a really long time before we can put sensors on those!
The weight isn't the whole story, it's how it is placed that matters as well.