some of the best 4.0 guys I know play 7.0, 8.0 and 9.0 all at 18+. I can name about a dozen guys doing this.
They play 4.0, then Mixed 7.0, 8.0, 9.0 then Combo 7.5, 8.5 and 9.5 combo and Tournaments, then play 4.0 and 4.5 in the fall.
They are solid 4.0s who make it to state on multiple teams. Several of them will be playing 8.5 and 9.5 Combo representing SC at sectionals
These guys play 7.0 Mixed and face similar players in playoffs and and state.
Based on my experience, a 3.0 guy/4.0 will not present any challenge for these type players.
dizzle is spot on in his assessment.