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

where

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!

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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

rearranging

C2 = C1 * (P2/P1)

solving

C2 = 28mL * (4.0atm / 1.0 atm) = 112mL

why do you think the answer should be 28mL?