Effects of heat pump capacity?
It depends heavily

Because ultimately, you have a room with heating/cooling loads, which you're trying to meet with the heat pump. 18 sq. m. is approximately 200 sq. ft., and I've generally seen 30 BTU/hr/sq. ft. as a rule of thumb for rough sizing. With this guideline, you'd need 6 kBTU/hr, or 1.75kW. Because your heat pump is going to be twice this, it's unlikely you'll notice an impact.

Why it depends

The problem with 30 BTU/sq. ft. is that is makes quite a few assumptions about your HVAC loads. If your insulation is especially poor, you will need more HVAC power. If you live in a milder climate (ex. 20-30 °C all year) you might not need any HVAC capability. There are too many variables for us to definitively say how you will be impacted.

How to determine the impact

If you found your old unit was running constantly, that's a sign that it's incapable of keeping up with your heating and cooling requirements. If, however, it was nearly always off (or, if it used an inverter, it was running at very low power) then it was likely significantly oversized. If you don't remember, try looking back at previous electrical bills. Unless all your appliances are also electric, or you drive an EV, you should be able to tell approximately how much power your old unit used. For a ~4kW unit, you should expect 1-2kW of electrical draw while it's operating (at full power).