Since you refer to this as her S Corp, I assume that she is the only shareholder.
[Ignore the paragraph that follows, which is entirely wrong due to confusing the employee elective deferral limit with the employer profit-sharing limit, and see my later comment. I won't correct this paragraph since doing so would put your follow-up comment out of context.]
Her maximum elective deferral is 25% of the $4K that she paid herself as compensation (assuming that combined with any elective deferral to the new employer's plan here total elective deferrals do not excess $18,000). That 25% must be excluded from the amount reported in box 1 of her W-2 and shown with code D in box 12 of her W-2. The S Corp can also make an employer profit sharing contribution.
The total of employee elective deferrals and employer profit-sharing contributions for the year cannot exceed 100% of her $4K of compensation. These combined amount of these contributions would be deducted on the S Corporation's tax return (Form 1120S).
