What was the reason why he came into the market?
If he had planned to hold it long term, and believes in it. Then why worry about what the price will be the next day... You're in it for the long run so put it aside and wait some more

Only a paper loss as long as he doesn't sell. It is much easier to hold than get mentally traumatized by selling at a low price only to find out that the price pumps the next day. Show him the the past graphs of bitcoin, it might ease his mind knowing that every year, it closes at a price higher than the previous year.

Any investment (not just bitcoin) should be done with the intention of holding long-term. Trying to time the market is a fools game and will lead to massive losses always.

Tell him to hold it for now , since the price isnt that good. And ask him not to worry about it. Bitcoin price is so unpredictable so if he hold it for the mean time theres a chance that he can get a higher profit in the future.

I advice to your dad that hold it and sell it when the price is good enough. Tell him that investing is a risk and if he want to earn higher profit he must not afraid to risk. Give him some information so that he can get enough knowledge about bitcoin.

4 BTC, that's a decent stash! Tell your dad to definitely HODL and to take a break if the price is causing him too much stress. Just believe in the tech and you will be fine. Something Tom Lee has said is that a majority of Bitcoin's gains come within a period of 9 or 10 days. So even if you are red during a majority of the year, those 10 days you do HODL you will be way up. Convince him to HODL, because if he sells at a loss, he will be hating it even more when the price soars back to $20k.

this is a fantastic post.  Selling at a loss can be quite traumatizing, I'm still kicking myself for selling at a loss before and the memory still burns in m my mnd.

Do explain to your father that it is normal to have fluctuations in the value. Advice him to have the patience and hodl as this can give him profit after some time.

Bitcoins holding is the best way your father can experience a return on his investment so I will advice you to admonish him to continue holding till a specific time before selling it off.