Here are some options:
Move on to unit 4 after Lesson 21 - Digital Assistant. You’ll miss some more advanced topics - more advanced loops and array processing - but students have programmed enough after lesson 21 to successfully build a program that meets the requirements of the Create PT.
Lesson 25 is a good stopping point if only because there is an assessment that covers lessons 22-25, but you might consider stopping after Lesson 23, as lesson 24 starts a new topic entirely.
There are ways to shorten Unit 4 and get a decent experience - and I would try as it might make for a good big-picture wrap-up to the course (big data, privacy, security, etc.)
I could make some recommendations about how to carve out a custom unit 4 that fits your needs as best as possible, but it depends how much time you actually have though. My first-pass recommendation would be do lessons Unit 4 lessons 10-14. If you wanted to add to that, I would consider doing lessons 1-4 minus the data tracker project.
Feel free to ping me directly (using @baker) from the forum or email me baker [at] code.org if you prefer.
Code.org CSP Team