If you have 4mm of grade 5 Ti firmly bolted to the side I would drop the HDPE all together! LF3 had 3mm of ti on it and I dont think anything really got through. Little hitter has 3mm G5 on the front and 1.5mm on the sides. But theirs is sloped quite heavily.

You could keep maybe 6mm on the side to absorb some impacts and slow weapons before the reach the Ti but 4mm of grade 5 is more than most machines have.