[Paul M]

A few things to consider.

There are currently a lot more vB4 sites then vB5.
However, many of them have been around a long while and dont need new styles.

Since vB4 isnt being developed anymore, any new style you create is unlikely to require many changes for future releases.
Again, since vB4 isnt being developed anymore, its user base will dwindle over time, however, atm, many people still prefer to start a new site with vB4 rather than vB5.

Since vB5 is still being developed, any vB5 style you create may need extensive changes when a new version is released (for example, many responsive changes in 5.1.7).