We store most of our data in our phones or online in some cloud based storage. And there are numerous ads while we are surfing on the internet. And if by any chance the ad has some virus or is put up by an hacker. He can easily ge our IP address by which he can get into our phones and collect all the stored information in it. Everything! And that is how most of our wallets or accounts get hacked!

Hacking someone's account can be done by the professional hackers. They have the knowledge, skills, gadgets and GUTS. That's why you should change your passwords very often.

i have lost over $4000 already and it's still counting!

Since you say you don't save the complete private key , it must have happened while using the private key to log in to your wallet .
Maybe you used a fake website to log in , do you use bookmarks for Myetherwallet or do you use google to find the site.
At least reinstall your OS and try to find out what exactly went wrong.

I have lost my myetherwallet recently. I have never used any wifi connection or give my private key to another. I just used the Imtoken application and boom, got hacked. I think everybody should use a virgin computer and cellphone and just for this job only.

As others people said, I follow the possibiliy of a web browser failure.

Do you know that is possible to create offline transactions with MEW ?  

https://myetherwallet.github.io/knowledge-base/offline/making-offline-transaction-on-myetherwallet.html

This could help you in the future to be protected against web browser attacks.

In my situation, I have an offline computer (this computer is NEVER connected to the network), I have the website in local storage and I follow the steps as described on the link.

This is quite impossible to hack you, as your private key is used on the offline computer, and if you use a dedicaced usd key for transfering the transaction information, and if you format it every times you use it, I think it becomes really hard to hack you  

I take care to delete the partition on the USB drive before every time I plug it again on the offline computer, to avoid possible viruses transferts.

Anyway, you could use an hardware wallet, but you still have this possibility if you don't want to  

Phishing sites is commonly used in Hacking of people's account by hackers there are several look-alike myetherwallets created by hackers to steal unsuspecting users password it is advise-sable to always keep your private key on a paper.

yea its possible to hack your account it can be hack because you store your private key online or you are this type that keep every information about your account online, the hacker may decide to send you a virus as soon you click on the item send to you every information of the account will be open to the hacker.

Wow! With this your level of security, it is still a surprise a hacker got through to your account. I might still want to believe probably you might have had your private key divulged at to a phishing site or probably sent it out wrongly. I don't know the  level of security of exchange sites that do request for one's private key.