My 4 month old cat is deep in heat and thowing herself at my 4 month
old mini shnouzer. Is it possible for him to penetrate her, and can I
feel safe leaving them in a room alone together?

My mother's American Eskimo (Spitz) dog once tried to mate with my
mom's Siamese cat. Both animals were injured during the struggle. I
suggest that you not leave these two pets together unsupervised.

Hi lucky teresa

This is quite an amusing question you have placed there. I imagine the
problem is certainly less amusing for you than for the reader. I have
been a practicing veterinarian for 15 years and have never come across
a case where penetration has actually happened. However, apparently it
is possible on rare occasions, as we have read in the comment above.
Only in heat does a cat tolerate to be mounted. Pregnancy is not
possible, though.
Since heat in the cat does occur frequently the only solution is to
have your cat spayed, which is easily possible at this age. Your cat
may be living indoors only but she will be living for the moment she
can escape through a door or window and, believe me, she will soon
return from her trip pregnant. Once she is spayed heat cycles cease
and the dog will not stand a chance to get anywhere near your cat's
back.