The wife saves $100/week x 4 weeks/month = $400 a month.
Together, the wife and husband save $400/month + $225/month = $<<400+225=625>>625/month.
In 4 months, their total savings is $625/month x 4 months = $<<625*4=2500>>2500.
They are going to invest $2500 / 2 = $<<2500/2=1250>>1250.
So, they can buy $1250 / $50/share = <<1250/50=25>>25 shares of stocks.
#### 25