Good question.
Let's start with some clarification.
The "4%" sounds like the 'magic number' for how much can be withdrawn safely from a pot of 60% S&P 500 index fund and 40% bonds/bond fund and not run out of money after, say, 30 years. (All before taxes--we are talking portfolio survival here. What you do with the money--pay taxes, eat, etc.--is up to you after you take the 4%/year out.) You called it "4% rate or return". Well, if you can get 4% consistently on average from all of your portfolio (whatever it is made up of), then they are the same thing. It makes 4%/year and you take out 4%/year and you do not outlive your money. You have won the game!
Now, if you already have 10% of your pot in a fixed investment yielding "nominal interest", say, 1.5%, then the rest of your portfolio (the other 90%) would have to earn an average of 4.28% to come up with an overall rate of return of 4%.
Will the other 90% earn 4.28%? What is it invested in (asset allocation)? Different asset categories have different long-term historical performance (we are always looking in the rear-view mirror here; nobody can read the future). The details matter. An S&P index fund with a high annual fee company will hurt you. (Say, 6% average annual returns minus 2% management fees = 4% average annual return, which is less than the 4.28% you need to not run out of money.)
I hope this helps.