I am sorry to hear of your predicament and thank you for finding my articles is of interest to you. Unfortunately the aircon specialist that you contacted is correct. More than likely, the cooling capacity of the system is insufficient to cool down your entire flat. A system 4 configuration does not mean that it will cool down 4 areas.
There are certain calculations that needs to be applied to sum up the minimum cooling capacity of an enclosed space. This is by no means exhaustive. As a guide, a space of about 4m x 3m with a ceiling height of about 2.8m would require about 9 200 Btu to cool down the area. (Btu : British thermal unit is used for the measure of power in steam, heating and air conditioning industries.) This will be a typical bricked room with a door and some windows. Not a glass house! I shall not get into further details as it will then be exhaustive.
Which brand you decide is a personal decision. (marketing and branding perhaps) What you should look out for is the cooling capacity of the outdoor Condensing Unit (CU).
For example, the cooling capacity of the CU is 22 000 Btu. You connect 3 indoor Fan Coil Units (FCU) each with a cooling capacity of 9 000 Btu. Now, when you turn on the 3 FCUs, the total load required is 27 000 Btu. However, the maximum cooling capacity of the CU is only 22 000 Btu. Thus, each FCU will not run at full capacity of 9 000 Btu but at about 7 300 Btu. If the CU is connected to another 9 000 Btu FCU, at full load, each FCU is actually running at 5 500 Btu. In short, the capacity of the CU is crucial ! You need to check the technical specifications of the aircon system prior to committing.
For your new apartment, what I would recommend is to first establish the areas you want air-conditioned and then work out the minimum cooling capacity. With that information, you then source for the air-conditioning equipment.
If you'd buy me lunch at your local deli, I can assist you to size up your cooling capacity requirement! Throw in a beer, I'll go shopping with you for your air-con system!
Hope the above helps.