What does it mean when a game is a 4X game

It is the VGA grade assigned to that item. You were probably looking at an item related to video games. This is a grade assigned by "Video Game Authority".

The VGA Gold level consists of the grades 100, 95+, 95, 90+, 90, and 85+. A very small percentage of items submitted to VGA receive a Gold level grade. An items flaws must be very minor, subtle, and can often be difficult to identify with the naked eye. A collector who is extremely condition sensitive should be satisfied with the condition of a Gold level item in the vast majority of instances.

You can find more details on the following website: http://www.vggrader.com/faq.aspx

Hope this helps.

"Metroidvania" is a term used to describe a sort of exploratory 2D platforming game. It comes from the original games of the Metroid series and some of the middle-era of Castlevania games (most notably "Symphony of the Night", which is what primarily gave birth to the term).

The most defining feature is usually the world. Rather than have set stages or a world map, the entire game takes place inside one giant map, which you must traverse about. Not just any map, but typically a very complex map filled with all kinds of secrets. The map is typically divided into sectors, more in an aesthetic fashion to denote that the denizens and dangers will be different. Sometimes you have teleports to assist you in travel, but other times you just have to remember where to go in the map.

There's various elements at play in the exploration. There's typically tidbits hidden in spots, or puzzles that have to be returned to when you find appropriate equipment. The exploration is also non-linear - your limits of progression through the map are by various tools or powerups that you need to acquire, but the order in which you must acquire some of these tools is not set. And in many choices, there are many paths to get those tools, or even get past obstacles without those tools.

A very popular element of these games, especially in the Metroid franchise, is the concept of "sequence breaking". Using special tricks or otherwise mastering the utility of your basic abilities, you can sometimes access items and areas far earlier than the game expects you to.

As far as I know, Metroidvania games have always been 2D platforming games. Aside from the elements of exploration, the actual gameplay of Metroidvania games vary widely.