Windows 7 Starter is limited to a max 2GB of usable RAM, so yes your extra 2GB is useless.
Sources:
Я использую Win 7 Starter 32-bit на Acer Aspire, 2 Гб оперативной памяти.
Сможет ли 2 дополнительных ГБ ОЗУ сделать мою систему быстрее?
При установке оперативной памяти свойства системы говорят, что в системе 4 ГБ, но только 2 можно использовать. Значит ли это, что мои лишние 2 ГБ бесполезны?
Windows 7 Starter is limited to a max 2GB of usable RAM, so yes your extra 2GB is useless.
Sources:
Many 64-bit CPUs/memory controllers support so called dual channel memory access to speed up memory accesss, if you have two sufficiently similar memory modules properly installed in a mother board that support this feature. See Wikipedia article on more the more general concept of multi-channel memory architecture for more details. So while the operating systems that you are currently using implements this artificial limitation for maximum usable memory, there might still be slight performance boost depending on the application.
However, as mentioned in below comment by @gronostaj, if your system is running out of physical memory, this added memory bandwidth is going to help you even less noticeably.
You need to consult your motherboard user manual for details how to install the memory to benefit from the feature.