Can you turn a RPI into a modem?

Ok, here is my idea. I have it raspberry pi B+ and what I want to do is is to use my 4g hotspot to give connection to the RPI and I want to be able to plug in the Ethernet cable in order to give connection to an old computer. Do you think it is possible? I don't have a modem, I only have my smartphone and this computer won't get my connection through it.

Is this by any means possible ?