In Japan, certain numbers are lucky and unlucky. It's really important to know these numbers because if you don't, you could be accidentally telling someone you'd like them to suffer a slow agonizing death when you're giving them omiyage (お土産, souvenirs) from your trip to Kyoto.

Death, agony and suffering

First, let's get the bad ones out of the way. Four is an unlucky number in Japan because it sounds like shi (死 death). This is why there are two readings for the number four, shi and yon. Whenever possible, people try to avoid using the deathy one.

The same is true of the ku (九 nine), which sounds like ku (苦 suffering, agony or torture). Similarly to four, there are two readings for nine ku and kyu.

Some buildings such as hospitals don't have fourth or ninth floors, although I've never personally encountered one. Maternity wards may not have a Room 43 because it sounds like shisan (死産 stillbirth).

Certain license plate numbers are not used such as 42, which sounds like shini (死に to death); 49, which sounds like shiku (敷く to run over); 42-19, which sounds like shini iku (死に行く to go and die); 42-56, which sounds like shini-goro (死に頃 time to die); and 24, which can be nishi (二死 two deaths or two out if you're a baseball fan).

Some of the Yakuza's scarier members use their license plates to express their contempt for their own mortality by choosing 4444. That's quite a bit of death and a car with this plate is one you don't want to cut off on the highway.

You have to get creative when you have a number that sounds like death.
For me the numbers do not mean suffering, but trying to learn all the counters in Japanese might be.