XSLT Loop - Four nodes at a time

Firstly I am aware of this question: XSLT: Loop selecting two elements at a time

However I have not found it to work due to the element structure or I just fail with using mod, one of the two.

<input>
  <node>
    <id>1</id>
    <value>3</value>
  </node>
  <node>
    <id>1</id>
    <value>3</value>
  </node>
  <node>
    <id>1</id>
    <value>3</value>
  </node>
  <node>
    <id>1</id>
    <value>3</value>
  </node>
  <node>
    <id>2</id>
    <value>4</value>
  </node>
  <node>
    <id>2</id>
    <value>4</value>
  </node>
  <node>
    <id>2</id>
    <value>4</value>
  </node>
  <node>
    <id>2</id>
    <value>4</value>
  </node>
</input>

I have the following layout of XML which has the following structure: - Nodes of the same ID will ALWAYS be grouped together - There will always be four nodes to one ID

I wish to be able to select the four nodes of one ID at a time and loop through each group of four, so that I can manipulate the data into one output line.

What would be the best way to approach this?

This XSLT:

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">

<xsl:key name="keyByID" match="node" use="id"/>

<xsl:template match="/">
    <output>
        <xsl:apply-templates/>
    </output>
</xsl:template>

<xsl:template match="input">
    <xsl:for-each select="node[generate-id()=generate-id(key('keyByID',id)[1])]">
        <block>
            <id>
                <xsl:value-of select="id"/>
            </id>
            <value>
                <xsl:value-of select="value"/>
            </value>
        </block>
    </xsl:for-each>
</xsl:template>


</xsl:stylesheet>

applied to your Input XML:

<?xml version="1.0" encoding="UTF-8"?>
<input>
<node>
    <id>1</id>
    <value>3</value>
</node>
<node>
    <id>1</id>
    <value>3</value>
</node>
<node>
    <id>1</id>
    <value>3</value>
</node>
<node>
    <id>1</id>
    <value>3</value>
</node>
<node>
    <id>2</id>
    <value>4</value>
</node>
<node>
    <id>2</id>
    <value>4</value>
</node>
<node>
    <id>2</id>
    <value>4</value>
</node>
<node>
    <id>2</id>
    <value>4</value>
</node>
</input>

gives this grouped Output XML:

<?xml version="1.0" encoding="UTF-8"?>
<output>
<block>
    <id>1</id>
    <value>3</value>
</block>
<block>
    <id>2</id>
    <value>4</value>
</block>
</output>

The output is grouped by <id>. Is that what you are looking for? I am not sure. This Muenchian Grouping just simpliefies your structure.