The first thing that you need to ask is "do I need an expansion memory module"?
On Tyros 4 an expansion module is only used to store any new external wave samples that are used to make a custom wave voice, and there is no alternative to installing expansion memory to use these. The module is not required for style, registrations, MIDI or audio songs!
Note that on a Tyros4 you can also make custom voices just from the wave samples that are already included in the keyboard's sample ROM, and you do not require an expansion module to make and store these.
Styles from any source (even Yamaha Premium Packs) do not require expansion memory, although in the latter case, styles included in Yamaha Premium Packs may use expansion voices contained in the pack, and so would require expansion memory to load these voices!
If after reading the above, you decide that you do need expansion memory, the only problem that you are likely to have with the Yamaha FL512M (apart from maybe regretting later that you did not get a module with more memory), is when you realise that you could have bought a Mutec equivalent (FMC-05) for £75 (GBP) less!
Similarly the larger Mutec FMC-06 equivalent to the Yamaha FL1024M (1 GB) module is £90 GBP less that what Yamaha ask for their product. I have had a Mutec FMC-06 fitted to my T4 for 7 years without problems.
Finally, be aware that flash expansion memory management on the Tyros4 is considerably more restrictive than on the Tyros5 and other keyboard's that use the YEM system. The T4 OS imposes limits on various parameters associated with custom wave expansion voices, the most troublesome being the so called 'Wave Count'. These limits can lead to you being unable to load further large Yamaha Premium Voice packs even though there is sufficient empty memory available on the expansion board for the pack! BTW trying to get sensible information prior to purchase from the Yamaha MusicSoft website about the requirements for these parameters for specific Yamaha Premium Packs is like trying to get blood from a stone!
Regards
Ian