Is DDR4 RAM compatible in a DDR3 RAM slot?

No, DDR3 and DDR4 are not cross compatible. The physical sockets are different among other things like input voltage, pin count, etc. See the picture below for a full size DIMM, which would be similar to a SODIMM.

For nyxee's question, DDR3L is different from DDR3 and DDR4 as it can work on a voltage range. The DDR3L RAM consumes lesser voltage compared to the DDR3. DDR3L is a dual voltage capable memory SoDIMM, which supports operation at both 1.5V and 1.35V.DDR3 works on 1.5v only while DDR4 consumes 1.2v only.DDR3 RAM uses a 240-pin connector, while DDR4 RAM uses a 288-pin connector. For more information go through this link.