// In event handler
Block block = event.getBlock();
if(block.getType() == Material.STONE) { // Check if block placed was stone
	boolean found = true;
	Block newBlock;
	for(int i=1;i<=4;i++) { // Check +1-4 in x direction
		newBlock = block.getRealtive(i, 0, 0);
		if(newBlock.getType() != Material.STONE) {
			// If any of them are not stone, break and declare false
			found = false;
			break;
		}
	}
	// At the end of this, if all the blocks were stone, "found" will still be true.
	if(!found) {
		// Otherwise, try in the opposite x direction (-1-4)
		found = true;
		for(int i=1;i<=4;i++) {
			newBlock = block.getRealtive(-i, 0, 0);
			if(newBlock.getType() != Material.STONE) {
				found = false;
				break;
			}
		}
	}
	// Continue for +1-4 z and -1-4 z
	if(!found) {
		found = true;
		for(int i=1;i<=4;i++) {
			newBlock = block.getRealtive(0, 0, i);
			if(newBlock.getType() != Material.STONE) {
				found = false;
				break;
			}
		}
	}
	if(!found) {
		found = true;
		for(int i=1;i<=4;i++) {
			newBlock = block.getRealtive(0, 0, -i);
			if(newBlock.getType() != Material.STONE) {
				found = false;
				break;
			}
		}
	}
	if(found) {
		// 4 in a row
	}
}