Sorry for your loss. You may wish to use the services of a professional. Filing a final return for a deceased person may not be straightforward if the estate is complex. Nevertheless, if you are comfortable learning what needs to be done, it isn't rocket science – and you've discovered already that Canada Revenue Agency (CRA) provides some guidance. You can call them if questions come up during the process.
To answer your specific question:
Yes, you are correct that the employer is required, by law, to send out an employee's T4 slip no later than the last day of February the following year, or the next business day if the last day of February falls on a weekend.
However, in a case such as this, earnings can typically be known in advance of year end, and it is possible to get a T4 issued early. In fact, CRA has a guide for employers, Filing the T4 Slip and Summary (PDF) which suggests, on page 18:
If an employee leaves, we suggest you calculate the employee's
earnings for the year to date and give the employee a T4 slip. Keep
our [CRA's] copy of the slip and include it with your T4 Summary when you
file it on or before the last day of February of the following year.
So, it is both permitted and feasible for your wife's employer to issue her final T4 early. If you contact your wife's company HR department to request that, I would hope and expect they would be understanding and helpful, considering the situation.
One more related tip, if you haven't done so already: Make sure any earnings or benefits owed to your wife's estate by the company (or their insurance plans) have been paid out, such as regular pay for the final pay period worked, quarterly profit sharing (if applicable), accrued but untaken vacation time (usually there is some), not-yet-reimbursed employment expenses (check her credit card statements, if she typically incurred work expenses), etc.