I agree with Rodney, the meaning of a card in a reading depends on many things; the question, the position, the other cards, the context etc..
When I'm thinking about card meanings I find that looking at the number and the element really helps as well as looking at the image on the card.
Think of the number 4 and the element of water and imagine how they interact.
Fours are about structure, boundaries, pauses, consolidation and foundations whilst water is an element that flows, reflects, has depths etc.
If you think of water being contained (as it is in the number 4), you may get more of a feeling for this card. Water represents the emotions, relationships etc and it likes to flow; contain it for too long and it becomes stagnant. That's what you get in the 4 of Cups.
This is a card about emotional stability but with that you have to beware of becoming bored and stagnant... You know that time in a relationship when the initial enthusiasm has started to wear off? The love is there but it's getting a bit stale and needs a bit of input (that cup being offered). The danger is that you're so stagnant that you won't notice the offered cup.
This card could be saying that you need to make sure that you don't miss opportunities or that you should look out for opportunities. It could also be saying that things are fine now but you need to watch out that you don't get stuck in an emotional rut.
Does that help?