Your ISP is providing you with a 4 Megabit per second throughput. There are 8 bits in a byte, so your actual download speed should be about 0.5 Megabytes per second (or 512 kbps).
However, there are many factors that can slow down internet traffic, normally always from latency in hardware.
If your download speed is 160 KB/s, you should first see if other devices on your local network are experiencing the same problem. If not, look into the hardware (network card) or software on your device to see if there is something going on there that is slowing down the network speed.
If all the devices on your network are at 160 kb/s, check out your LAN setup. If you have multiple routers or are using a WiFi, investigate upgrading or troubleshooting those bottlenecks. A good way to test this is to directly connect your computer to your ISP's modem. If your speed is faster, then your local LAN hardware is bottle-necking your speed.
If you still have slow speed even with your computer connected directly, it's time to call your ISP and crush some skulls.
Difference between bandwidth and actual speed?
Imagine that your connection to the internet is like a river. Bandwidth is the width of the river. If you have a tiny creek of a river, then not much water goes through at one time. If you have a massive, mile-wide river, more water can flow through it at once.
Speed is closely related to bandwidth. The wider (bandwidth) your river is, the more data you can access at once, and, obviously, faster.
If I put a bunch of water-sucking machines along the river, it still is just as wide, but each machine will suck less water down the line, because they are "eating all the bandwidth". The water is divided among the clients, and each client will take longer.
Translate that analogy to your network - there is something along the "river" that is narrowing it - your ISP maybe, but probably a router. There could also be devices on your network that are sucking data and slowing you down.
But I receive only a speed of 160 kbps when downloading files over internet
Sometimes I get much less, sometimes I get much more. You need to run a proper speed test to find out what your actual maximum speed is, not just a few random files because those may have been from slow servers or during times of heavy load.