How can I be sure that I have to install four sensors on my van

Can be a sensor, Oxigen Sensor, Heating Sensor. Try going to Auto Zone.

These relays tend to be in a fuse/relay panel under the hood on the firewall or inner fender wall. Auto Zone should have the Chiltons/Haynes repair manual for your auto that covers the make/model and year. It's a good book to have at any rate. It will tell you where the fuse relay panels are and what they contain.

Replace O2 sensor, this isnt expensive and generally solve the problem; if you note the code number, send it us.

Well, for starters they should have never told you to change the oxygen sensors. The codes they pull are only a guide and it is up tp a tech to find the underlying problem. It sounds like you have a catalytic convertor issue.

yes it can be. how many rpm is with 10 mph in tachometer? Also can be faulty oxigen sensor and air flow sensor, And can be faulty catalysator.

go to nearest auto zone they have the tool to replace your o2 sensor

they should have told which O2 sensor was out...bank 1...bank 2...etc!!

Check with Autozone or O'reillys to name a few. There is such a thing as a captive part (dealer only). Another alternative is the Salvage yard. The latter is really easier than you would think, most of the time they have already pulled the part, will pull the part, or you can do it and learn where it is and how to remove it. In general you may need a special socket for O2 sensors.
As part of the Emisson devices it would have a longer warranty, something like 8 years and 100,000 miles. Check owners manual, call Corporate customer service.

Speedometer sensor is usually in the cars computer. Sounds like it's electrical though. Could be a sensor part of the alternator. Call Advanced or auto zone. Some places can test stuff like that for free.

remove the oxigen sensor and drive it, if it get so much better power then you catalitic at exaust is restricted and have to be replace, note: by driving whit 02 sensor remove it will make a lot of noise don't be worry this is normal just make sure nothing overheat by the 02 sensor hole