Serial data transmission need a framing. When sending ASCII data, you have e.g. CR/LF termination to indicate a data entity. In binary transmission, you need other kinds of delimiters, e.g. unique start of frame and end of frame symbols. Unfortunately they can't be easily distinguished from same data bytes, so you need e.g. to use escape sequences.
In other words, your question is falling short. Before you consider how to transmit a single float value, you need to know how the transmission start can be uniquely marked.
A very simple scheme to transmit only a single number can be an interframe delay. Guarantee that before and after the 4 binary characters nothing is transmitted for e.g. 10 or 100 ms.