Known as the Lord of Blended Pleasure and aligned with the Moon in Cancer, the Four of Cups represents emotional riches, love and tenderness, but it can also mean emotional uncertainty.
The Moon is at home in Cancer, and as the Moon 
represents feminine, yin energy that is changeable, Cancer represents comfort, support, home and healing.
The problem is that often this directs one's gaze back to the past, the good old days, so it can be a card of dwelling in the past and missing what is right here, right now and more real than past remembrances.
In more traditional decks, this card is often depicted as four cups presented as a choice or gift, with one cup often spilled or just out of reach. This points to the meaning behind the card - that often we have love and affection within our reach but it is the the dream that is out there, that we yearn towards and not being able to reach, causes us pain. It can even point to a temptation of the heart, such as a flirtation or an affair.
What we fail to understand is that we have three cups already full to overflowing, and the cup that is just out of range is full of promises that may or may not be tangible. So we often ignore the love and pleasures that we do have for the false promise of the unattainable.
The real problem appears to be a familiarity, or boredom with life and love, a sort of feeling of "Is that it?" that fills one with a kind of hopelessness and depression. It is a result of wallowing in the watery feeling depths of Cancer muddled with the inconsistency of the Moon that gives one the feeling of drowning in ones problems, but it need not be this way.
It is time to throw some cold water on your face, get out in the sunshine and take a fresh look at your resources - those of the heart - and start to appreciate the three full cups you do have.
If you are mourning past losses and feeling that nothing can be as good as what you had, then it's time to snap out of it - today is here, and you are missing all the wonderful things in your life that you could be enjoying by putting your energy into those instead.
Image from "Altered Tarot" © flickr.com/photos/kirapoet