Since the problem does not mention compounding, I will assume that the account is simple interest.


Actually, the months are not standard units to measure time intervals, because some months
are 30 days, while other months are 31 day, and even there is the month February,
which may have 28 or 29 days.


Since the problem does not point the concrete months, it means that for this problem 
it does not matter, which months they are.  

So, I will count each month as 30 days.


I also will forget about leap year, which inevitably should be present at this time interval.


So, I will edit this problem as a typical mathematician does.


    +---------------------------------------------+
    |     As mathematicians like to joke:         |
    |   A telegraph pole is a well-edited pine.   |
    +---------------------------------------------+


So, in this context, 4 years 5 months and 23 days is the same as  4*12*30 + 5*30 + 23 = 1613 days, or

     = 4.480555556 years.


Now I will use the formula for the simple interest 

    I = P*r*t.


Here I is the interest, given as  4,200 in this problem;  P is the principal, which is 50,000;
r is the unknow rate of interest and t = 4.480555556 years.


Thus, I substitute these values into the formula and get

    r =  = 0.018747675,

which I reasonably round to  r = 0.01875,  or 1.875%.


ANSWER.  In this problem, the interest rate is about 1.875%.