Application of the 25% profit sharing rule

It's my understanding that profit sharing is limited to 25% of your plan compensation. Is this limit for specifically profit sharing, or all employer contributions? Specifically, does safe harbor count towards that limit? I have a client who has a solo K plan and made about $100,000 and maxed out deferral. Her document is a 4% Safe Harbor NE. Can she contribute 4% as a safe harbor contribution and then 25% as profit sharing, or can she only do 21% profit sharing to hit a limit of 25% comp as employer contributions?

This sounds odd to me because a solo 401k plan, by its nature and legal structure, does not (usually?) have the safe harbor feature that is found in traditional 401k plans because those rules ensure that the plan is not discriminating between the employees and HCEs or owners.  Solo 401k plans are specifically structured for owner-only businesses without full-time employees (except the owner(s) and possibly their spouse(s)).  If the owner(s) hire full time employees, they may want to implement a plan with safe-harbor features so they could contribute the safe harbor contributions without testing but that wouldn't let them provide an additional 21% nonelective contribution just for the owner(s) as any contributions in excess of the safe harbor would have to be tested under 401(a)(4).  

That said, back to your actual question, annual additions paid to a participant's account cannot exceed the lesser of:  100% of the participant's compensation, or $70,000 (assuming <age50) for 2025.  But, as you state, there is also the deduction limit where an employer’s deduction for contributions to a defined contribution plan cannot be more than 25% of the compensation paid (or accrued) during the year to eligible employees participating in the plan.  Since the 25% limit is on the employer's deduction for contributions to the plan it does not apply to the salary deferrals as the deduction for those deferrals is really for wages paid.  See 404(n).  

So with $100K compensation, assuming these are W-2 wages, she can contribute $23,500 (assuming <50) and she as employer can contribute an aggregate employer contribution of $25K in 2025.  Note that I am assuming these are W-2 wages.  If her compensation is actually earned income, there will be a difference (see Self-employed individuals: Calculating your own retirement plan contribution and deduction | Internal Revenue Service).  Also, this example assumes only one owner and no spousal compensation.

For FWIW I see a lot of 'solo-k' plans with safe harbor in them, particularly SH to NHCE only. In the event an employee is hired and becomes eligible, the SH is already built into the plan document without a need for an amendment. 

Similarly - I don't like to see them with no service requirements/ immediate entry. A better design is the maximum 1 year of service, age 21, semi-annual entry etc so that if someone is hired who isn't the owner, there is time for an outside party to review the plan provisions before there are any errors, such as a missed opportunity to defer. 

If someone is hired that they want to let in sooner, usually its not a big deal to do an amendment to change that. 

Don't forget about Long Term Part Time employees.  LTPT employees may become eligible for the plan under the LTPT rules, even though they may not satisfy the plan's eligibility of 12 months / 1,000 hours.