Gradient based Distribution

Ok brainiacs !!

I think I'd better get some help before I have a brain anurism :)

I have been strugling with this for days ...ok only one ... and I do
not have a mathmatical background ( at least this is the excuse I am
sticking with )

I am trying to calculate monthly sales figures to indicate trend based
on 3 years average quarterly results to get a monthly pattern of sales
eg.
Q1 10,200
Q2 22,700
Q3 15,900
Q4 11,300

In my humble opinion I think I need to calculate each months figure
based on a distribution of the sales for that quater and the gradient
of the present quarter's sales to the next, with the last quarter
linking to the first,but i just cannot come up with the equation !!

Please help anyone,I am trying to complete a business plan and i'm
sure this can;t be rocket science.

Many thanks in advance

.