Final analysis of proton form factor ratio data at<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mi>Q</mml:mi><mml:mn>2</mml:mn></mml:msup><mml:mo>=</mml:mo><mml:mn>4.0</mml:mn></mml:mrow></mml:math>, 4.8, and 5.6 GeV<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mrow/><mml:mn>2</mml:mn></mml:msup></mml:math>

Precise measurements of the proton electromagnetic form factor ratio $R={\ensuremath{\mu}}_{p}{G}_{E}^{p}/{G}_{M}^{p}$ using the polarization transfer method at Jefferson Lab have revolutionized the understanding of nucleon structure by revealing the strong decrease of $R$ with momentum transfer ${Q}^{2}$ for ${Q}^{2}\ensuremath{\gtrsim}1$ GeV${}^{2}$, in strong disagreement with previous extractions of $R$ from cross-section measurements. In particular, the polarization transfer results have exposed the limits of applicability of the one-photon-exchange approximation and highlighted the role of quark orbital angular momentum in the nucleon structure. The GEp-II experiment in Jefferson Lab's Hall A measured $R$ at four ${Q}^{2}$ values in the range $3.5\phantom{\rule{4.pt}{0ex}}{\text{GeV}}^{2}\ensuremath{\le}{Q}^{2}\ensuremath{\le}5.6\phantom{\rule{4.pt}{0ex}}{\text{GeV}}^{2}$. A possible discrepancy between the originally published GEp-II results and more recent measurements at higher ${Q}^{2}$ motivated a new analysis of the GEp-II data. This article presents the final results of the GEp-II experiment, including details of the new analysis, an expanded description of the apparatus, and an overview of theoretical progress since the original publication. The key result of the final analysis is a systematic increase in the results for $R$, improving the consistency of the polarization transfer data in the high-${Q}^{2}$ region. This increase is the result of an improved selection of elastic events which largely removes the systematic effect of the inelastic contamination, underestimated by the original analysis.