If the mp4 file is small, then you could email it to yourself.
If you have GoodReader, then you could WiFi or USB it to the iPad and then transfer it to oPlayer..
There is a program that most all iPad users have called GoodReader. It's the 'Swiss Army Knife' for the iPad and it can store files all together and in folders - or however you want them - and can transfer files to and from your PC using either WiFi or USB without the need for iTunes. It's built in web-browser can download files too, something that Safari doesn't offer - except for PDF format. From within GoodReader you can open those files in any app that supports that type of file. Having said that, though, when a compatible app does open that file it makes a local copy that it works on, so the original copy in GoodReader is left untouched. If you want to store the modified file in GoodReader you have to transfer it back there by some means.
GoodReader can also attach several files to an email and then send them using the iPad's native Mail app - something you can't do from within Mail itself. It also gives you the opportunity to annotate PDF files and save the resulting file so that it can be read on a PC or Mac.
Tim
