I'm making an order form for a quick sale of multiple products. 

There are 9 products in total but the offer is for the customer to select any 4 of them for the price of 2. I've got a script for ensuring the user doesn't check more than 4 checkboxes but I can't find a way to ensure that the submit button is disabled if they have selected less than 4 items.

here's an example of the script:

```
<SCRIPT LANGUAGE="javascript">

function KeepCount() {

var NewCount = 0;

if (document.B2G2F.os1.checked)
{NewCount = NewCount + 1;}

if (document.B2G2F.os2.checked)
{NewCount = NewCount + 1;}

if (document.B2G2F.os3.checked)
{NewCount = NewCount + 1;}

if (document.B2G2F.os4.checked)
{NewCount = NewCount + 1;}

if (document.B2G2F.os5.checked)
{NewCount = NewCount + 1;}

if (document.B2G2F.os6.checked)
{NewCount = NewCount + 1;}

if (document.B2G2F.os7.checked)
{NewCount = NewCount + 1;}

if (document.B2G2F.os8.checked)
{NewCount = NewCount + 1;}

if (document.B2G2F.os9.checked)
{NewCount = NewCount + 1;}

if (NewCount == 5)
{
alert('Pick Just FOUR Please');
document.B2G2F; return false;
}
}
</SCRIPT> 

```

and the html:

```
<div id="choose" style="background-color:none; margin-top:5px;"><form action="https://www.paypal.com/cgi-bin/webscr" method="post" target="_top" name="B2G2F">
<input type="hidden" name="cmd" value="_s-xclick">
<input type="hidden" name="hosted_button_id" value="################">
<table>
<tr><td style="padding-top:5px;">
<h3 style="color:#FF9000;">Simply select your FOUR DVDs from the checklist and then click the buy now button</h3>
</td></tr>
<tr><td>
<input type="checkbox" name="os1" value="X" onClick="return KeepCount()"> Choose woodworking hand tools
<input type="hidden" name="on1" value="choosetools"><br/>

<input type="checkbox" name="os2" value="X" onClick="return KeepCount()"> How to sharpen woodworking hand tools
<input type="hidden" name="on2" value="sharpening"><br/>

<input type="checkbox" name="os3" value="X" onClick="return KeepCount()"> How to use a Chisel
<input type="hidden" name="on3" value="chiseling"><br/>

<input type="checkbox" name="os4" value="X" onClick="return KeepCount()"> How to use a Bench Plane
<input type="hidden" name="on4" value="bench plane"><br/>

<input type="checkbox" name="os5" value="X" onClick="return KeepCount()"> How to use scrapers and scraper planes
<input type="hidden" name="on5" value="scrapers"><br/>

<input type="checkbox" name="os6" value="X" onClick="return KeepCount()"> Learn Dovetailing
<input type="hidden" name="on6" value="dovetailing"><br/>

<input type="checkbox" name="os7" value="X" onClick="return KeepCount()"> Make piston fit drawers
<input type="hidden" name="on7" value="drawers"><br/>

<input type="checkbox" name="os8" value="X" onClick="return KeepCount()"> How to Hammer Veneer
<input type="hidden" name="on8" value="hammer veneer"><br/>

<input type="checkbox" name="os9" value="X" onClick="return KeepCount()"> Learn French Polishing
<input type="hidden" name="on9" value="polishing"><br/>

</td></tr>
</table>

<input type="hidden" name="currency_code" value="GBP">
<div style="float:right; margin-top:12px; margin-right:352px;">
<input type="submit" value="BUY NOW" id="submit" name="Submit" class="button" alt="PayPal — The safer, easier way to pay online.">
<img alt="" border="0" src="https://www.paypalobjects.com/en_GB/i/scr/pixel.gif" width="1" height="1">
</div>
</form>

```

I've been able to find plenty of answers for a single checkbox but the multiple checkbox problem solutions I've tried haven't worked at all.

Ideas?