class Bookandgo_AutoSku_Model_Observer
{
    public function autoGenerateSku($observer)
    {
        $product = $observer->getEvent()->getProduct();
        $sku = "TA1000";
        $product->setSku($sku);
    }
}

public function catalogProductNewAction(Varien_Event_Observer $observer)
{
    $product = $observer->getProduct();

    $products = Mage::getResourceModel('catalog/product_collection')
        ->addAttributeToFilter('attribute_set_id', $product->getAttributeSetId())
        ->addAttributeToSort('entity_id', Zend_Db_Select::SQL_DESC)
        ->setPage(1, 1);

    $sku = filter_var($products->getLastItem()->getSku(), FILTER_SANITIZE_NUMBER_INT);
    if ($sku) {
        $sku = $sku + 1;
    } else {
        $sku = 1000;
    }

    $prefix = 'DF';
    if ($product->getAttributeSetId() != 4) {
        $prefix = 'XYZ';
    }

    $product->setSku($prefix . $sku);
}

$prefix = 'DF';
if ($product->getAttributeSetId() != 4) {
    $prefix = 'XYZ';
}

<?php
class Bookandgo_AutoSku_Model_Observer {
  public function autoGenerateSku($observer) {
    $productColl = Mage::getResourceModel('catalog/product_collection');
    $productColl->addAttributeToSort('entity_id', 'DESC');
    $productColl->setPage(1,1);
    $lastproduct=$productColl->getLastItem()
    $lastproductAttributeSetName= Mage::getModel('eav/entity_attribute_set')->load($lastproduct->getAttributeSetId())->getAttributeSetName();
    $lastSku='';
    if($lastproductAttributeSetName=="Default"){
      $lastSku = ltrim($lastproduct->getSku(),"DA");
     }
    elseif($lastproductAttributeSetName=="Tour and Adventure"){
      $lastSku = ltrim($lastproduct->getSku(),"DA");
    }

    $newSku=$lastSku+1;

    $product = $observer->getEvent()->getProduct();
    $newproductAttributeSetName= Mage::getModel('eav/entity_attribute_set')->load($product ->getAttributeSetId())->getAttributeSetName();
   if($lastproductAttributeSetName=="Default"){
      $product->setSku('DA'.$newSku);
     }
    elseif($lastproductAttributeSetName=="Tour and Adventure"){
       $product->setSku('TA'.$newSku);
    }

  }
}