> Can I use both in the same computer?

Your system is only compatible with DDR4 memory.  The layout of the pins on a DDR3 modules are completely different to that of a DDR4 module.  **You would be unable to install the DDR3 modules into your motherboard, if you attempted to do so, you would damage the module and/or the motherboard itself.**

> [![enter image description here][1]][1]
>
> DDR4 SO-DIMMs have 260 pins instead of the 204 pins of DDR3 SO-DIMMs, spaced at 0.5 rather than 0.6 mm, and are 2.0 mm wider (69.6 versus 67.6 mm), but remain the same 30 mm in height.

Source: [DDR4 SDRAM][2]


  [1]: https://i.sstatic.net/2DFNG.png
  [2]: https://en.wikipedia.org/wiki/DDR4_SDRAM