Heating 2% solution to get 4% solution
The best approach would be to crystallize $\ce{ZnSO4.7H2O}$ and then weigh the required amount and prepare a fresh solution.

How:

Slowly evaporate the solution until it is about one-fifth of its original volume. Caution: Do not boil the solution as it may spit.

Allow the concentrated solution to cool until crystals form.

Filter off the crystals and put the filter paper and crystals on a watch glass and dab dry with another piece of filter paper.

Allow it to dry thoroughly (if you're keen on very accurate concentrations)



I assume by $x\%$ solution you mean $x\%$ weight($\pu{g}$)/volume($\pu{mL}$)
You may proceed to prepare the solution by dissolving $4$ grams in 100 $\pu{mL}$ water. In case you need a different volume, you may use unitary method.
Edit:
Crystallization is certinly more tedious than simple evaporation and volume restoration, but is bound to be very accurate. There might be loss of salt in trace amounts during evaporation, adheration to vessel walls et cetera. But if much precision is not your requirement, you may follow the latter, by all means.