Since the BL15 is used, (3600kv) I would try the 4 cell. The fan is 6 blade? My HET 6904 on 4900kv runs a 3cell, and the 600 watt inrunner does the 950 watts WOT nicely in actual use. I use a 3700kv outrunner on a 6904 rotor on 5 cell for about the same watts. A regular 5 blade fan unit, similar to a Wemotec mini on a 4100kv outrunner runs 1000 watts on a 4 cell. Could be less if it were well balanced.

Actual figures on my Rafale using Wemotec Mini fan pro, 4100kv Cyclone ourunner and 4 cell 40c the fan rpm is 48,000+ rpm, using the Super Brain data logger. (The fan is rated for 45,000 rpm)

I admit to pushing the limits on the fan units, however, after flying a similar aircraft in an underpowered manner is asking for a crash. Also, since the vertical stabilizer incorporates no rudder, I strongly suggest adding it on. Run a pushrod down the dorsal or plant a servo right into the fuselage. (Hacking) Seriously, getting down the runway straight on nosegear steering only, is asking for an aborted takeoff, or very short flight off the side of the runway, ripping the wheels off the plane and causing instant re-kitting.

Before anyone decides to build an EDF jet, they should be cautioned. Spinning up a fan to 30,000 rpm or more is dangerous, and also, balancing the fan unit, if the factory hasn't, is crucial. If you won't balance the fan, use the stock setup and 3 cells and deal with the 1.7 lbs. on a 3 pound plane. I like to suggest that with any high-wing-loading aircraft, a high thrust to weight ratio is strongly called for.