D4:
- More flood
- Smaller, lighter
- 18650 cell
- Gets hotter faster
- Compatible with a clip
- Super-floody optic available
D4S:
- More throw
- Bigger, heavier
- 26650 cell
- Takes longer to heat up, can sustain higher output
- Newer UI with more features
- Has aux LEDs
- Tail magnet available
The LED choice also affects how floody or throwy it is… but pretty much anything in a D4 will be pretty floody, and almost anything in a D4S will be somewhat throwy. A D4 with the floody optic isn’t quite as floody as a mule (bare emitter with no reflector), but it’s close. So for a flooder I’d suggest picking an emitter mostly based on other factors, like color temperature, tint, heat, and efficiency.