More than one device distribution of transfer issue

Hello. First, i hope this is right place to ask this question. But straight to business.

I’m using 5.25 version of MikroTik and i have 4Mb/s transfer. We have lots of devices connected to our MikroTik. While there is one computer connected, everything is fine, but when second or third device become connected (just connected - no transfer using), internet slows down a lot. We did a small experiment. My phone was the only device connected to MikroTik and i started to download some data with full speed 400kb/s. Then someone else connected with his laptop. Windows manager on this laptop show no any transfer, but downloading on my phone drop a lot!

Okej, the story is long but someone can help with this issue? I’m green with MikroTik. Maybe there is need for simple configuration or something?