Although freshly charged NiMH measure at 1.4 or so volts, they quickly drop to 1.2 under load. However, unlike alkalines, they stay around that 1.2 for a long time (this is called a "relatively flat discharge curve").

Alkalines by contrast have a much steeper discharge curve but do present a higher terminal voltage when new. (Hint: You can separate unused alkalines from somewhat-used ones with a digital multimeter. The unused ones should measure 1.6V or so.)

Either way, your flash seems to be absurdly sensitive to battery voltage. No doubt the reason your flash "uses up" the alkalines so quickly is that they have dropped below the voltage the flash is happy with. There's likely still plenty of energy in them.

Conclusion: Maybe the flash is defective but I bet it's just a poorly designed unit. I have never had a flash that failed to work very well with 4xAA NiMH cells. This includes camera-maker-brand flashes "dedicated" to my camera, other-maker flashes also "dedicated" to my camera, and some "vintage" Vivitars that just do "auto-thyristor" mode with their own sensor.

Aside: I wouldn't trust Ikea brand batteries of any type. Nor any other store brand. Big chain stores pay the real battery manufacturers to wrap their store name on whatever batteries are cheapest this month. otoh I've had absurdly good results from not-nationally-advertised-brand alkalines, and far less leakage than from one of the best-known name brands. Your results may vary.

For NiMH cells, from my experience, I'd try Panasonic (Eneloop or not, your choice, Eneloops are low self-discharge but lower energy storage), Tenergy, or Powerex (Maha). Powerex/Maha also make some of the best NiMH chargers available.