The Pearl Gecko reference manual states for Power Configuration 2 (DC-DC) of the EMU chapter: "IOVDD could be connected to either the main supply (as shown below) or to VDCDC, depending on the system IO requirements." If I connect IOVDD to VDCDC, will a pin reset still wake the device from EM4 Shutoff (EM4S)? This seems unlikely, since the DC-DC is not available in EM4S so VDCDC will be unpowered.
The integrated DC-DC on EFM32 Pearl Gecko is indeed unavailable while in EM4S. However, VDCDC is still powered while in EM4 Shutoff. In your example, IOVDD will also then be powered in EM4S, so a pin RESET is still detectable.
This result arises from the fact that in EM4 Shutoff, the DC-DC converter configuration is reset to its default Startup configuration (DC-DC converter disabled, bypass switch is on), in which the VREGVDD input voltage is directly shorted to the DC-DC converter output through an internal switch. Consult the datasheet for more information on the Bypass switch impedance specification.