There shouldn't be any major issues with such combination given that your device supports that combination. Try to get them from the same brand and you should be fine. In fact, my ASUS N550JV, oddly tho, came with exactly that installation: 4GB + 8GB.
Considering dual-channel memory architecture, if you want to use it, it's recommended that the memory modules should be of same frequency due to operational conditions.
Modules rated at different speeds can be run in dual-channel mode, although the motherboard will then run all memory modules at the speed of the slowest module. Some motherboards, however, have compatibility issues with certain brands or models of memory when attempting to use them in dual-channel mode. For this reason, it is generally advised to use identical pairs of memory modules, which is why most memory manufacturers now sell "kits" of matched-pair DIMMs. Several motherboard manufacturers only support configurations where a "matched pair" of modules are used. A matching pair needs to match
Edit
Since your laptop (Dell Inspiron 15) specifications say that
Memory
- 4GB 1 DIMM (1X4GB) DDR3L 1600Mhz
- 2GB 1 DIMM (1X2GB) DDR3L 1600Mhz
- 6GB 2 DIMM (1x2GB+1x4GB) DDR3L 1600Mhz
- 8GB 2 DIMM (2x4GB) DDR3L 1600Mhz
1x4GB + 1x8GB will not be suported. Either get a 2GB or 4GB module for upgrade. In case your manual differs than this general Inspiron 15 one, if you don't see a 12GB there, do not upgrade to 12GB. I believe values between 8GB and 16GB are not intended to be supported given that the manual says the motherboard supports 8GB and 16GB.