So if I only have 1,5GB RAM, then I won't be able to play TS4? Even if I set the graphics to low?

It is possible the game will work with 1.5GB of RAM. The minimum requirement is 2GB though:

REQUIRED: Internet connection required for product activation
OS: Windows XP (SP3), Windows Vista (SP2), Windows 7 (SP1), Windows 8, or Windows 8.1
PROCESSOR: 1.8 GHz Intel Core 2 Duo, AMD Athlon 64 Dual-Core 4000+ or equivalent (For computers using built-in graphics chipsets, the game requires 2.0 GHz Intel Core 2 Duo, 2.0 GHz AMD Turion 64 X2 TL-62 or equivalent)
MEMORY: At least 2 GB RAM
HARD DRIVE: At least 9 GB of free space with at least 1 GB additional space for custom content and saved games
VIDEO CARD: 128 MB of Video RAM and support for Pixel Shader 3.0. Supported Video Cards: NVIDIA GeForce 6600 or better, ATI Radeon X1300 or better, Intel GMA X4500 or better
SOUND CARD: DirectX 9.0c Compatible
DIRECTX: DirectX 9.0c compatible