You need alot more than 4. I dont even know how you earned just 4. Usually to create a pack of 3 cards, it costs anywhere from 400-800 gems.
The cards are used to create badges. When you get a full set of cards to a game, you can turn them into a badge, losing the cards.
The badge gets added to your profile, gives you 100 Steam xp, which mainly increases your maximum friendlist allowed, but also can give you profile displays. You also get a steam emote and background based on the game its for, though its random which specific ones you get.
If you dont care about that stuff, then its not really important, but if you like collecting stuff, or sprucing up your Steam profile, they are worth getting a bit into, though I dont recommend spending real money on it. You can trade cards with other users, usually best to check the 'Trading' section of the specific game's forum.