The word Mississauga has the following multiplicity: $M(1\times), I(2 \times ), S(4\times), A(2 \times), G(1\times), U( 1 \times)$
You want a word with $4$ letters, so lets take for example $(M,I,S,A,G,U)$.
For a $4$-letter word you have a lot of combinations. Now you just have to write them down.
I'm going to do a few and let you do the rest.
For example if you have a $4$ letter word with one $M$ , one $I$, one $A$ and one $U$. You have $(1,1,0,1,0,1)$. For this you have $\frac{11!}{1!1!1!1!}$
If you take one $M$, two $I$ and one $S$ , you have $(1,2,1,0,0,0)$. For this you have $\frac{11!}{1!2!1!}$
You have to write all of combinations possible of $(M,I,S,A,G,U)$ with the sum value of $4$, where $M,I,S,A,G,U$ can vary up to their multiplicity respectively. In the end you sum up everything and get your result. However the word Missisauga has alot of combinations, so it might take some time.