Which schools are you applying to? Each school values different sections of the DAT differently. If you are applying to UofA, focus on the MDT and PAT, and less so on bio/chem.
That being said, writing the DAT in 4th year should offer you an advantage for bio/chem as you have likely taken all your prereq's. PAT probably has the steepest learning curve, so I would focus on that this summer. Try giving 30-60 mins a day to practicing PAT over the summer, combined with occasional review of bio/chem topics that you are less confident with. Reading comp is reading comp, its tough to get better at. Try reading articles from scientific american, nature, etc. with your morning coffee to get more comfortable and quick reading technical pieces.
As for study materials, I used DATdestroyer and DATbootcamp exclusively for the Canadian DAT and found it relevant. I've also heard good things about DATcrusher, but haven't used it myself.
Good luck and feel free to message me if you have any questions!