Welcome, Newcomer. First, stick and module are terms used interchangeably. Meaning that stick = module, or module = stick. To address your concern; however, mixing RAM is not a good idea. If you mix RAM, you'll be bound by the following limitations:
Where you'll find problems with the above mentioned three are as follow:
The voltage of the mixture must be set at the higher of the two. Meaning if the 4GB stick requires a voltage of 1.7V, but the 1GB stick requires a voltage of 1.5V, you have to find out if the the 1GB stick can operate safely at 1.7V. If the 1GB stick cannot operate at 1.7V, but you set your DRAM voltage to 1.7 anyway, you'll risk damaging the mobo, and possibly the CPU.
Like voltage, the RAM timing can only be set to accomodate one of these two sticks. Let's say your 4GB stick has a CL5, but your 1GB stick has a CL7. In this instance, you'd have to set the timings to CL7 because the 1GB stick cannot operate at the faster timing of CL5.
Finally, when it comes to frequency, this one is less dramatic, but still equally as important. Generally, you'd have to set your frequency to the lowest common value between the two. Meaning if the 4GB stick can operate as high as 800 MHz, but the 1GB can only go as high as 533 MHz, then you'd have to set your DRAM Frequency to 533 MHz.
As you can see, mixing RAM is not a good idea because of the physical limitations each module or kit has.