Old design had 4 engines and 3 water-jets. Number of engines do not dictate number of water-jets. Often that one waterjet (center) configured as a booster.
And with gas turbines one gets acceleration, for efficiency, both diesel+diesel or diesel+gas are not efficient above the cruise (or high-cruise) speed. Since in diesel + gas or diesel + diesel, only sideward waterjets and the engines (diesels in both) connected to them is enabled, both becomes identically efficient, hence invalidating the arguments.
At high seas and high sea-states, gas-turbines give a bigger hand.
Admiral Tatlıoğlu a year ago has delivered a speech on TTHB and told that they are slimming the design down in favor speed and agility. Where does it fit among these, whose words we are going to follow? I will honestly wait official renders (instead of MPAC and old TTAB design) and lay-outs. I haven't commented much on his words back then since i also have known that a particular division of Navy favors sea-keeping abilities and is in favor a larger platform, while another group was conceptualizing TTHB + USV packs.
At this point while i have explicit details on project, i would recommend people to be cautious and wait for official disclosure. Since information leaking from individual sources within companies, offices may not reflect the truth.