is this a bad opening for white?

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Solidous

Chessroshi

I don't think so. b5 looks really fishy to me, as it seriously weakens the queenside. I think maybe a developing move instead of 5. a4 is in order as it will highlight blacks centre deficiency better.

Scarblac

No, it is a main line. Kasparov has played it as White (edit: in the move order 1.d4 d5 2.c4 c6 3.Nc3 dxc4 4.e4 b5 5.a4)