Q: WiFi speed drops continuous at 4 seconds

Try logging into your router or wifi AP and changing the channel the wifi sends signals on. You could also try connecting to 2.4 GHZ if the wifi connection supports this. Most likely it is an issue with interference from another wifi connection.

You can also try pinging your router continously (open terminal, ping the router's ip). To find your router ip open your sytem preferences, network and look for "default gateway" under your wifi connection. If you ping this address post the output here. Maybe you're getting high ping time already all of the time, or you're not and the connection just shuts on you.

Either way, first check your router, changing channel wil hopefully make a difference.

Open a terminal window and enter ping 192.168.0.1

Output should give you a ping to your router with a response time of that ping. This will show you how many packets get lost when the connection drops and also if you have a high latency (or not) to begin with. If the latency is high to start with it could very well be a problem with the router. Do you have other devices that also lose wifi connection? Or just your macbook?

You can also put your wifi adapter in "diagostics mode" (see below) it will gather some data that could point you to the problem.

About Wireless Diagnostics - Apple Support

Lastly. Did you recently upgrade your macbook to Yosemite? Maybe your network prefences got screwed somehow. Erasing /Library/Preferences/SystemConfiguration should help with this. Or straight up erasing the network from your "known networks" and connection a new.

Well it doesn't seem to lose connection. So, I most likely the problem is with your router, especially since another device seems to give you the same problems. Try resetting the router or doing a factory reset. Maybe the manufacturer has fimrware updates available, you could try those.

If those also don't resolve the problem I would look into getting a new router.You could still try a different wireless network (campus, starbucks) and see if the problem persists, but like I said, the problem is most likely with the router.