yes, it will be busy. with 4 days in the parks, you might want to judge the standby times and try to navigate using the regular express passes the first day. if you're at the parks a half hour before opening, you can get in and hit a few attractions before any significant wait times start to build.
The wait times build between 10-12. From 12-7 (with a 10pm park close) is very busy, with 12-5 being the busiest of all. Don't be surprised to see 60 min waits everywhere and the free express being done for the day between 11a-1p for rides such as MIB, ROTM, Hulk, and Spider-Man.
Express plus will cost you over $30 per person during that time...