only try to install apps that are compatible with oC4

lib/installer.php

@@ -130,10 +130,19 @@ class OC_Installer{
 
                 // check the code for not allowed calls
                 if(!OC_Installer::checkCode($info['id'],$extractDir)){
+			OC_Log::write('core','App can\'t be installed because of not allowed code in the App',OC_Log::ERROR);
 			OC_Helper::rmdirr($extractDir);
                         return false;
 		}
-		
+
+                // check if the app is compatible with this version of ownCloud
+		$version=OC_Util::getVersion();	
+                if(!isset($info['require']) or ($version[0]>$info['require'])){
+			OC_Log::write('core','App can\'t be installed because it is not compatible with this version of ownCloud',OC_Log::ERROR);
+			OC_Helper::rmdirr($extractDir);
+                        return false;
+		}
+
 		//check if an app with the same id is already installed
 		if(self::isInstalled( $info['id'] )){
 			OC_Log::write('core','App already installed',OC_Log::WARN);