It's the same. 4GB of DDR3 is the same as 4GB of DDR4. There is no difference in capacity. In terms of speed, you can't make a comparison in that way. There are many variables including frequency, bandwidth, latency, the architecture as a whole. It's really not comparable, more like apples to oranges.
Having 12GB is really only beneficial if you have a triple channel architecture and that is not common these days. Most consumer systems will be dual channel. Most HEDT or enterprise platforms will likely be quad channel.
If you plan on upgrading to a consumer platform, then it would be wise to stick to a 2 x8GB, 2 x16GB, 2 x32gb or whatever capacity you require. What memory to target will depend entirely on what platform and exactly...