I would start by reading Dr. Nicki Newton's work "Math Running Records." It is absolutely a game changer for your classroom. It takes a lot of work up front, but will give you information to help target specific missing elements of the different operations.

Some other great tools that I have used are Reflex Math (grants are available: https://reflex.explorelearning.com/grant?_gl=1*zd6c2v*_up*MQ..*_ga*OTA0NjA4MjQ4LjE3MjQzNDQwNDQ.*_ga_S0NLYBJCR0*MTcyNDM0NDA0NC4xLjAuMTcyNDM0NDA0NC4wLjAuMA..) if your school doesn't have access to Reflex. A free website to go to is XtraMath which also gives students daily practice.

I would incorporate daily fluency instruction for students, no matter what level they are. It can look different for different groups of students but it will pay off in dividends!

I would suggest hanging up some anchor charts or giving the student a small reference sheet. Teach how to refer back to these resources. Start small and build from there.

Two thoughts on operations.
1. Counting is foundational in order for students to be able to add/ subtract and multiply divide. If a student is struggling, check their counting!
In order for students to successfully add/ subtract, they must have solid mastery over the forward and backward number sequence.
For students to multiply and divide, they must be able to skip count forwards and backwards.
If they are struggling, add counting into your remediation. When students are more confident and master the counting sequences they are able to start using more efficient strategies when learning to use the 4 operations.

2. If students are struggling, you may need to incorporate more concerete activities into their learning. This applies to both addition/ subtraction AND multiplication/ division. Students in 3rd and 4th grade do not get enough hands on experiences with multiple groups and equal sharing in order to develop conceptual fluency.
The Recovering Traditionalist has an excellent video regarding fact fluency! Here is the link:
https://www.therecoveringtraditionalist.com/how-do-children-build-automaticity-in-math/

I use story telling and connection making to guide the students. Many students do not understand that they have already used these basic math operations in every day life. For example, telling the story of a person having a sleepover with three friends and sharing a pizza with eight slices. By doing this, I have the students write out how to solve it through pictures or words. This reinforces the ideas in math and the real life situations they have every day.

I have had my students to use colored cubes to represent two numbers. This helps with addition and subtraction. I also teach my students how to use a number bond because it is important that they know the whole number and the parts of an equation.

To support students struggling with basic math operations, use hands-on manipulatives like blocks or counters to help them visualize the concepts. Break down problems into smaller steps and provide frequent practice to reinforce learning. Incorporate math games to make practice more engaging, and connect math to real-life situations to increase relevance. Pair students with peers for collaborative learning, and use positive reinforcement to build their confidence and encourage progress.