Simplify a circuit with current sources
The 4 and 2 ohm resistors are not in parallel so you can't combine them that way. 

Instead you can convert the current sources and their parallel resistances into equivalent voltage sources with series resistance to make it easier to reduce the circuit.

Use ohm's law to convert. For example the 4A source and parallel 4 ohm resistor is equivalent to a 16V source in series with that same 4 ohm resistor.