The difference is relatively minor so that alone wouldn't indicate a particular SEN.
My understanding re quantitative reasoning is that it's more problem-solving focused rather than pure calculations. Check GLL website - they used to give description/examples for these tests. So word problems, equations, sequences, etc. That said, fluency in mental maths/calculations helps massively. How good is he with that? Basically, having instant recall of tables (both multiplication and division) and number bonds is a big boost in standardized testing. Algebra (without calling it that) is often an important component - e.g. missing numbers, etc.
You can look at Power of 2 to help increase fluency. Schoefield and Sims Mental Maths books, CPG - I think they are called Maths Workout? It's been a while...
With regards to problem solving, an emphasis on word problems will help (there are books on Amazon called just that). Singapore Maths is a good resource as is Galore Park's textbooks - they used to be called Junior Maths, but I think the need edition is just Maths Year 3, Maths Year 4 etc.
Primary Maths Challenge is another resource for puzzle type problems, but probably better from Year 5.
CEM maths resources - not sure if they have anything for Year 4, they are mainly used for 11+ preparation.
WHSmith has a "Challenge" series as well as problem solving.