I was receiving errors from the bank system after having installed the uc_pasat4b module - http://www.ubercart.org/project/uc_pasat4b. After having used the sermepa system for every other website I have built, I was totally confused as to why it wasn't working and why the module wasn't sending all the data that I thought it should be. As I rolled my sleeves up ready to dive into the code and sort it all out, it turns out, after looking at the 73 pages long manual (!!!!), that this system requires you to enter configuration in the bank system (https://tpv.4b.es/consultas/html/default.html) rather than in Drupal like the Sermepa system.

Process for use...

1. Install the module
2. Go to admin/store/settings/payment/edit/methods and enter the "Código de Establecimiento" which is the same as the first field to enter into https://tpv.4b.es/consultas/html/default.html sent to you by the bank
3. Activate the module on the same page (I installed uc_conditional_payment so that only admin could see this payment type whilst testing because I was installing it on a live site)
4. Go to https://tpv.4b.es/consultas/html/default.html, log in using details given by bank
5. Click on "Modulo de configuracion" at the top of the screen the click "configuracion" in the left hand menu
6. Enter the following information into the following fields (note that the README.txt has wrong URLs

URL que devuelve el desglose de la compra:
http://www.XXXXXX.com/pasat4b/checkout_details (XXXXXX = your domain name)

URL que graba el resultado en la BD del comercio (TRANSACCIONES AUTORIZADAS):
http://www.XXXXXX.com/pasat4b/payment_ok

URL que graba el resultado en la BD del comercio (TRANSACCIONES DENEGADAS):
http://www.XXXXXX.com/pasat4b/payment_ko

URL de continuación posterior a la página de recibo
http://www.XXXXXX.com/pasat4b/return

URL de recibo (TRANSACCIÓN AUTORIZADA):
http://www.XXXXXX.com/pasat4b/payment_ok

URL de recibo (TRANSACCIÓN DENEGADA):
http://www.XXXXXX.com/pasat4b/payment_ko

Y pinchar "Actualizar configuracion"

7. Now we test the system by making a purchase with this payment type.
8. When entering bank card details in the test environment I found that the card number 12345 didn't work but the 40000 did. The date field just requires any date in the future and CVV2 is 111 (see manual)
9. When you are completely happy and are ready to activate the real system (and not the test environment) then navigate to the bank config page (see point 4 and 5 above) then click on "Paso a produccion" in the left hand nav.
10. Click on the "Pasar a produccion" button in the page

I hope this saves someone else lots of time!