It's not a cheap F2P version of 4th Ay unless you guys are talking about Lee being able to do 4 attacks in one turn but his durability is nothing compared to 4th Ay then I'm gonna agree with you but other than that, I'll have to disagree.

Lee SB Mystery, unlike 4th Ay Mystery, can be available for the whole 10 rounds because its cd can be decreased by 1 round every time Lee can do a 5-hit combo standard attack. At max SB, Lee gains additional 3 attacks and among those 4 attacks, Lee just needs to do at least 2 5-hit combo standard attacks to make his Mystery available in the next turn. If your Lee can do that for the whole 5 rounds, your Lee can do 20 standard attacks in 5 rounds. Add in also the 15% Crit boost, which might be very minimal, can still be boosted with other ninjas that add more boost to Crit (Tenten/Tenten GNW +15%, +30% total; Tenten SB +20%, +%35 total).

But things are different now though in 5.0. 4.0 Crimson's Guidance would have been a good passive buff for Lee SB because it boosts Lee's Atk +20%, and Combo +40%. With the Combo boost, Lee can do more 5-hit combo standard attacks and based on my experience here with my Lee and Crimson Fist on different game modes where I use Guidance and my current ranking in my server (ranked #16, S467), in my estimation, in 10 standard attacks, 8-9 of those where 5-hit standard attacks and rarely does a 1 hit standard attack happen. But with 5.0 and Guidance being nerfed to Skill Guidance, a Lee SB would probably be useless unless you have a very high Combo or doing those non player battles like Plot and Elite Instances where the stats of your enemies are low.