Ok, now I know what you are talking about, you mean 4 axles, one of which is a lift axle.
Example:
The only company that I know that has these are Promotex and it is on a Mack CH 613 and has a sleeper and cutting off the fifth wheel. That can easly be removed simply by popping it off and using a hobby knife to cut the fifth wheel off and then a dump body can be mounted. If you don't want macks, you will have to take the cab off the chassis (again, simply pop it off) and put your own cab on it, which may require some kitbashing to do. I love Macks and I would use the mack cab and just add the dump body, but you may have a different opinion and want to use another cab such as a Peterbilt, Kenworth......
Here is a pic of the lift axle mack
and can be found here:
http://www.promotex.ca/provisioner/catalogue/index.php?SearchFormSubmitted=true&SearchText=lift+axle
Here are the dump bodies available from promotex:
http://www.promotex.ca/provisioner/catalogue/index.php?AdminSearch=&SearchFormSubmitted=true&SearchText=dump
Also, Boley make coal dump trucks, they have a tall dump body wall to allow for larger loads of coal but they are only 3 axles and do not have a lift axle. You might be able to modify a lift axle in it but I have never done something like that to a boley truck so I couldn't tell you if it is easy or not, sorry. Here is a pic of a Boley Coal Dump Truck:
and the link to the page, there are GMC and International 4900 models available:
http://www.dept187.com/search.php?key=coal
Hope this helps you out some.