Change Customer Order #

You need to go in your phpmyadmin and change the increment value of the order_id column in the hikashop_order table with the query:
ALTER TABLE jos_hikashop_order AUTO_INCREMENT = 10000;

It's not related to HikaShop. You need to change that in your database using a database edition tool like phpmyadmin (most hosting companies use phpmyadmin). If you don't know how to do that, you should ask your hosting company which should explain you how to access your database. We can't tell you that as the method is different for each hosting company.

If you have a paypal business account, paypal will display the payment interface so that your users don't have to have a paypal account in order to pay for the order. That's not something handled by HikaShop. It's done automatically by paypal if you have a business account. If it doesn't work, you should ask paypal:

Business accounts allow you to do business under a company or group name, accept all payment types for low fees, and accept payment from customers without PayPal accounts.

It's not something you should do in joomla but in phpmyadmin. If you don't know how to do that, you should ask your hosting company which should explain you how to access your database.