Routes are conflicting with each other. Is it possible to set priority?

The priority are in what order you specify them. So place it in your priority order.

Code:

$route['fetch/(:any)/(:num)/(:any)'] = 'fetch/$1/$2/$3';
$route['fetch/(:any)/(:any)/(:num)'] = 'fetch/$1/$2/$3';
$route['(:any)/(:any)/(:any)/(:any)'] = 'main/user/$1/$2/$3/$4';