Matlab Simulink size mismatch

I am having some trouble using the typecast() function in Matlab Simulink. I am recieving an array of uint8 values (called data) from serial communication. I want to convert some of these bytes to a different data type. If I try to use typecast(uint8(data(1:4)),'single') and try to assign its output to a variable of type single, I recieve a size mismatch error. Any help is greatly appreciated.