When only two candidates advance, voters have limited choice in the general election. More voters participate in general elections than primary elections (of any type), and so what choices voters have in the general election matters. Under Top Two, most races feature only one Democrat and one Republican. A minority of races feature only two Democrats or two Republicans, meaning that even fewer voters will have a candidate in the race they support. Independents and "third party" candidates almost never advance a candidate under Top Two in elections where both Democrats and Republicans run.
Top Four would likely advance both a Democrat and a Republican in every race. Nearly every race would also feature more than one candidate from one of the two major parties. And a significantly higher number of races would include a candidate outside of the two major parties. This allows voters to hear a broader spectrum of opinion in the debates, and it ensures that many more voters will have the opportunity to vote for a candidate they support.
Most elections under Top Two still feature one Republican and one Democrat, even in heavily partisan districts, but under Top Two voters lack any alternative candidates. Only very rarely does Top Two advance two candidates from the same party, which may render a more competitive race in heavily partisan districts, but that comes at the cost of providing no choices outside of the majority party to any voters within that district. And often such races happen as a result of vote-splitting among a large number of candidates in the preliminary.
On the other hand, Top Four would advance two candidates from the majority party in nearly every race. It would also advance at least one candidate from each major party. And it would advance far more Independent and alternative party candidates. Top Four would thus accomplish the goals of Top Two better than Top Two while avoiding the pitfalls that have led to many opposing the system.
Under Top Two, most candidates are eliminated after the preliminary election, yet typically about half as many voters participate in that preliminary election as in the general election. Further, that small group of voters tends to be older, more conservative, and less representative of racial minority populations. Top Four would ensure that this group of voters would not have the power to eliminate otherwise viable candidates that the more representative general election voters may support.
In a vote-for-one preliminary election with a large number of candidates, votes may split up in such a way that neither the first or second-place candidates are representative of a majority of voters. For example, in California's Congressional District 31 in 2012, a majority Democratic, majority-minority district advanced only two white, conservative Republicans to the general election. This was because only those two Republicans ran, while four candidates ran as Democrats, splitting the Democratic vote too evenly among them. As another example, Louisiana's 2015 first round of election (note that although Louisiana does not use Top Two, the system operates similarly if no candidate wins a majority in the first round of election) eliminated all but one Republican, and that candidate was likely the weakest Republican in the race, ultimately leading to the election of a Democratic governor in an overwhelmingly Republican state.
With Top Four, any candidate receiving at least 20% of the vote in the preliminary election is guaranteed to proceed to the general election, and in fact candidates can expect to advance with far less than that. As a result, no viable candidate is likely to be left out of the general election. Then, the use of ranked choice voting in the general election ensures that vote-splitting will not affect the outcome of the general election.