Every machine in this room runs backwards. The pH came from a log, so the concentration comes back by ten-to-the-power: $[\mathrm{H^+}] = 10^{-\mathrm{pH}}$ — read aloud: H-plus equals ten to the minus pH. pH 4 means $1 \times 10^{-4}\ \mathrm{mol/L}$; the pH was the exponent all along, wearing its coat inside out.

Keep the minus on BOTH trips. Ten to the PLUS pH climbs the wrong way — it would say acids strengthen as pH rises, and every lemon disagrees. Sanity rail: higher pH, SMALLER $[\mathrm{H^+}]$. If a mild sample hands you a concentration above 1 mol/L, the sign fell off in transit.