Building Step 1
According to 42009 instructions 
Changes
- One steerable axle removed for better maneuverability.
- Front stabiliser moved towards base of superstructure:
  Position is (nearly) symmetrical between the two last axles and the supports.
- Transmission for stabilisers built more compact. Functing selection (in/out resp. up/down) by one lever as in the original model. 
- additional HOG steering as introduced by user efferman on eurobricks (http://www.eurobricks.com/forum/index.php?/forums/topic/85388-mod-42009-mobile-crane-mods-and-improvements/) by means of warning light.
(also posted here: http://www.doktor-brick.de/index.php/Thread/1301-42009-Modifikationen-und-Mocifikationen/?postID=19919#post19919)

Building Step 2
According to 42009 instructions
Modification
- Rear part changed to have transmission fron 42042, usings ideas from user yannmar (rebrickable MOC-1312, see http://rebrickable.com/mocs/yannmar/motorized-hydraulic-excavator):
  Functions:
  a) Boom raise (white, cf. image 017_84209)
  b) Boom extension (blue) 
  c) Winch (green)
  d) Forward/reverse rotation supplied for stabiliser operation (red, on side)

Target: No need to switch polarity on the battery box to reverse actuators.
All four functions can be selected in parallel.

Building Step 3
Crane arm from instructions of Ultimate 42009 model (see http://www.jurgenstechniccorner.com/index.php/moc-s/mod-s/ultimate-42009) with minor modifications.
Among other things I shortened the inner boom by two studs because I felt that the 'original' one sticks out too far on the front of the truck.
The hoist has a block and tackle setup with four wheels.
I am using about 10 metres of 0.4 mm string.

"Name" for this modification:
As this is 42009 model with 4 axles it resembles previous 8421 crane:
8 + 42009 => 8420.999
Also it needs to have "42" in it to refer to 42042 that has provided the gear box.

The initial goal was to use parts only from 42009 set. 
I quickly discovered that clutch elements from 42042 are easier to use than the original ones because of the odd length. 

Some enforcing structures in step 1 and 2 were made from parts that are available only from more recent sets. This allowed for smaller and simpler structures.