Why are iPhone4 and iPhone5 (iOS 7) not recognizing bluetooth devices? when I go to Settings -> Bluetooth and turn it on, devices (both iPhone 4 and 5, iOS7) start searching for devices with bluetooth, but never find anything. On the other hand, when I search from Android, iPhones are always found and can be paired, but sometimes not connected because of not supporting some devices (for iPhone 4 is written for me that Xperia Z2 is not supported). 

The problem is that I'm trying to discover nearby beacon, but as I've said, I cannot discover anything with iPhones. Android discovers it without problem. 

How can I solve it, so my devices find surrounding devices sending bluetooth signals?
iPhone 4 has bluetooth whereas iPhone 5 has AirDrop. So they are not compatible with each other. 

That is answer to a part of your question that iPhone doesn't recognise other iPhone (5 and 4).