Second attempt at Event 5 based on what I learned from feedback and judging other scripts.

Posh2013_Event5

function Select-UniqueClientIP {
  [CmdletBinding(SupportsPaging,DefaultParameterSetName="IPv4")]
	[OutputType([System.String[]])]
	param(
        [Parameter(Position=0,Mandatory,ValueFromPipeline,ValueFromPipeLineByPropertyName)]
		[ValidateNotNullOrEmpty()]
		[Alias("FullName")]
        [string[]]
		$Path,
		
        [Parameter(Position=1)]
		[ValidateSet("W3C", "IIS", "NCSA")]
		[Alias("Type")]
        [string]
		$Format = "W3C",		
		
        [Parameter(Position=1)]
		[ValidateNotNullOrEmpty()]
        [string]
		$Filter,
		
		[Parameter(ParameterSetName="IPv4")]		
		[Alias("v4")]
		[Switch]
		$Ipv4,
		
		[Parameter(ParameterSetName="IPv6")]	
		[Alias("v6")]
		[Switch]
		$Ipv6		
	)
	
	Begin
	{		
		$_accumulator = @()

		[scriptblock]$VersionFilter = {$_}
		
		if ($Ipv4)
		{
			$VersionFilter = {$PSItem.AddressFamily -eq [System.Net.Sockets.AddressFamily]::InterNetwork}
		}
		if ($Ipv6)
		{
			$VersionFilter = {$PSItem.AddressFamily -eq [System.Net.Sockets.AddressFamily]::InterNetworkV6}
		}
		
		switch ($Format) {
			"IIS" {
				$ClientIPIndex = 0
				$Delimiter = ','
				break
			}
			"NCSA" {
				$ClientIPIndex = 0
				$Delimiter = ' '
				break
			}
			default {
				$ClientIPIndex = -1
				$Delimiter = ' '
				break
			}
		} 
	}
	
	Process{
		$fileCounter = 0
		Foreach ($file in $Path)
		{
			$fileCounter++
			try {
				$stream = [IO.StreamReader]$file
			}
			catch 
			{
				Write-Warning -Message "Could not open $Path. $($_.Exception.Message)"
			}				

			$lineCounter = 0
			do 
			{
				$lineCounter++
				if ($Format -eq "W3C")
				{
					if ([char]$stream.Peek() -eq '#')
					{
						$ClientIPIndex = -1
					}
					if ($ClientIPIndex -lt 0)
					{
						$line = $stream.ReadLine()
						while ($line -notmatch "^#field")
						{
							$line = $stream.ReadLine()
						}
						$ClientIPIndex = [Array]::IndexOf($line.Split($Delimiter), 'c-ip') - 1
					}														
				}
				$line = $stream.ReadLine()
				try
				{
					$_accumulator += [Net.IPAddress]$line.Split($Delimiter)[$ClientIPIndex]
				}
				catch
				{
					Write-Verbose -Message "Could not interpret IP on line $lineCounter of $file. $_.Exception.Message"
				}
			
			} while (-not $stream.EndOfStream)
		}
	}
	
	End
	{
		#Send result to pipeline

		$selectParams = @{Property = @("Count", @{Name="IPAddress";Expression={$PSitem.Name}})}
		if ($PSBoundParameters['Skip'])
		{
			$selectParams['Skip'] = $PSCmdlet.PagingParameters.Skip
		}
		if ($PSBoundParameters['First'])
		{
			$selectParams['First'] = $PSCmdlet.PagingParameters.First
		}

		$uniqueResults = $_accumulator | 
			Where-Object -FilterScript $VersionFilter |
			Group-Object -Property IPAddressToString 
			 
		$uniqueResults | Select-Object @selectParams | Write-Output
		
		$count = 0
		if ($PSCmdlet.PagingParameters.IncludeTotalCount)
         {
             $TotalCountAccuracy = 1.0
			 if ($uniqueResults)
			 {
			 	$count = $uniqueResults.Count
			 }
             $TotalCount = $PSCmdlet.PagingParameters.NewTotalCount($count, $TotalCountAccuracy)
             Write-Output $TotalCount
         }
	}
}