Does anyone know if horizon 3 &4 requires SSE 4 and above to be able to run ?? I have the athlon ii x4 without SSE 4support. Thanks in advance !!

Not sure but there is a demo for FH4 so I suggest trying that. Needless to say as you’ve probably figured out, the CPU is likely to be too slow for a good experience even if it does work, but it won’t hurt to try the demo and find out.

If I download the demo and works can after purchase the game without downloaded again ??

You would have to download the game separately. But before trying to run before you can walk, why don’t you download the demo and see if and how it runs.

game and demo are 2 different things
if its a trial you wouldnt need to download again
do you have gamepass for PC, could try it that way too