WIthout Importing your Flow
04x00 (Hex) transtlates to 1024 in Decimal
Are you specifiying Hex or decimals registers when making the call ?
Its quite unusual for meters (although a lot of chinese manufacturers do not follow norms) to start at register Zero - usually they are in the 3000 or 30000 range
Can you post a picture of the modbus flexgetter config ?
Craig