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Will U4 run on older hardware?

I am talking quite old, DX9, GL 1.4, 512Mb Ram, mobile cpu, windows xp, I can upgrade to win 7 32 bit, does the OS have to be 64bit?

If not what is the absolute minimum hardware reqs?

No, that wouldn’t even run UDK.

512Mb RAM wont let you run even Windows 7 alone.

SYSTEM REQUIREMENTS:
Desktop PC or Mac
Windows 7 64-bit or Mac OS X 10.9.2 or later
Quad-core Intel or AMD processor, 2.5 GHz or faster
NVIDIA GeForce 470 GTX or AMD Radeon 6870 HD series card or higher
8 GB RAM

This is the min. system requirement that will let you work with acceptable framerate. The CPU is still not good enough for building lights though.

There is a thread you may want to look at where users provide their system specs and their experience running the editor.

Card that supports DX10 feaure set is absolute minimum to even think of running UE4