There's nothing that says you can't use array variables in the SHIFTIN/OUT command. As with everything, try it and see.

What you can do (a little secret you may not know about) is that you can use normal brackets () around your array variables rather than square ones to distinguish them from the square brackets in the SHIFTIN Command... like so...
Code:
ShiftIn FPU_DATAIN, FPU_SCLK, MSBPRE, [FloatByte(0), FloatByte(1), FloatByte(2), FloatByte(3)]
By the way, your example had a comma missing after MSBPRE and another after FloatByte[1] either of which whould have cause a compilation failure.