forward voltage for 350mA (per P, 700mA Total) will be (per datasheet)
Typical: 6.4V
But it could be lower
or it could be as high as 7.8V
forward voltage for 700mA (per P, 1400mA Total) will be (per datasheet)
Typical: 6.8V
But it could be lower
Or it could be higher
Scientifically, that literally is as close as you can plan it.. And you had better hope that the 4 die are well matched.
You would be much better served by running them in 4S1P and using a proper driver (probably boost mode, if you're planning on two cells)
LEDs are really poor candidates for Direct Drive, and they are also really poor candidates for Resistor Current Limiting. They're dynamic loads (based upon temperature and age) and they are not precision values (each LED WILL have a different Vf from the next one).
LEDs need an active circuit, be it an SMPS or a Current based Linear Regulator..
Ben