so first of all, Henry's law states the relationship between vapor pressure over a solution and solubility of a gas in solution is directly proportional...
P = k * C
P = vapor pressure of the gas over the liquid solution
k = a constant
C = concentration of gas dissolved in the liquid phase
That equation doesn't specify what the units of P and C are. The constants we use are where those units are specified. Example if we say our system is CO2 in H2O @ 25°C and lookup the Henry's law constant for that system, we'll find
KH = 29 atm/M
so that if we use that constant, the units will be atm for P and molarity for C.
however, we might have found the constant in other units. let's say we found
KH = 66.8 kPa / (g/L)
in which case the units of P would be kPa and C would be g/L
The relationship doesn't require specific units!
let's go back to your problem.. we know this
P1 = KH * C1
P2 = KH * C2
dividing to get rid of KH
P1/P2 = C1/C2
C2 = C1 * (P2/P1)
C2 = 28mL * (4.0atm / 1.0 atm) = 112mL
why do you think the answer should be 28mL?