I've bought two floureon 5500 mAh 4s batteries to place in series. Can I use XT60 connectors to connect the lot, or must I go with an XT-90?

Battery capacity (in mAh) is not relevant for choice of connector, neither is battery voltage (in volt).
Only current (in A) is relevant.
What current do you expact your ESC/motor to draw?

Or, futureproof, you go by what battery can deliver, for that you need to know both capacity and C-value.
Too big a connector is not a problem, just a minor weight penalty.

What (other) connectors are you using? Keep using same size, if possible, for uniformity?

At 2x4s you can expect some sparking, you may need anti-spark version of connectors.

Connector tests, plus pictures
translate...www.elektromodellflug.de/hochstrom-st.-bu..html
Older version of that page, contains some other connectors
translate...www.elektromodellflug.de/oldpage/hochstromst/hochstromstecker.htm

Excellent site, do have a look at the controller-, motor-, charger-, power supply-, battery- etc. tests.

Vriendelijke groeten Ron

Good response. Ecalc reported current draw at something in the neighborhood of 60 amps max. I will have to re-run the numbers to be sure. I'm a gentle flier so it is unlikely I'd stay at max current for long. The batteries are 35 or 40 c.

From a current point of view, I'd say stick with the XT-60s. Check occasionally after flight - if they are warm that would indicate a change to bigger connectors but with a peak near 60A and a lot of time below that I'd be surprised if they warm at all.

OTOH, if you have to do/redo the battery connection anyway and your batteries come with huge 10g leads, it might be worth stepping up in connector just for ease of soldering the thick battery leads into a bigger cup

I run XT60 series 2x 4S x 5300mah at 90 amps takeoff in my 1/4 scale 14 lb Bravo and no issue. The connectors have been tested at 100 amps continuous for 5 minutes and no issue as long as soldered correctly.

Thanks for all the guidance, team. I soldered the connectors together and all seems sound. When I get my receiver in tomorrow, I'll assemble the whole shebang and take a watt and power reading - this may take some doing as I've got no help and 1/4 scale stuff is not small or weak.

The Floureon batteries (2 x 4s) took a charge of 16.94v each! I'm working with almost 34 volts in series - just checked with the wattmeter.

Originally Posted by FOGeologist

Thanks for all the guidance, team. I soldered the connectors together and all seems sound. When I get my receiver in tomorrow, I'll assemble the whole shebang and take a watt and power reading - this may take some doing as I've got no help and 1/4 scale stuff is not small or weak.

The Floureon batteries (2 x 4s) took a charge of 16.94v each! I'm working with almost 34 volts in series - just checked with the wattmeter.

Unless they are "HV" LiPos, the charged voltage should max out at 4.20/cell or 16.80V for 4S. Might be worth a bit of checking; either charger settings or you measuring device