There are several reasons why your newly bought domain redirects you to Hostinger 404 error page or does not work at all:
You haven't added your new domain to your hosting account;
You use HTTPS to load your website without installed SSL;
Global DNS servers have to fetch new domain details and it might take up to 12-24 hours after domain registration for your domain to start working;
Your browser or local DNS are cached.
NOTE: You can learn how to clear browser cache over here or how to flush DNS cache here.