Convert SQL query to C# LINQ qu

Please, read my comment to the question first.

As i mentioned your query is not efficient due to the usage of [in] clause two times.

Let's try to improve your sql query. So, this:

select * from PatVisit p
inner join Device d on d.PattId = p.PatId and d.SyncDate is not null
where p.VisitDate in
( select max(VisitDate) from PatVisit pv where VisitId in (4,7,2,8,9) group by PatId )
and DATEDIFF(d,p.visitDate,GETDATE()) <10

can be replaced with this:

DECLARE @finaldate DATETIME
SELECT @finaldate = DATEADD('d', -10, GETDATE())

SELECT p.*
FROM
(
    SELECT p1.*, ROW_NUMBER() OVER(PARTITION BY p1.PatId ORDER BY p1.VisitDate DESC) RowNo
    FROM PatVisit p1
    WHERE p1.VisitDate >= @finaldate AND p1.VisitId IN(4,7,2,8,9) 
) p INNER JOIN Device d on d.PattId = p.PatId and d.SyncDate is not null
WHERE p.RowNo=1;

As you can see, i've used ROW_NUMBER() function[^], which is very useful to get only the first row from nested query.
Note, that WHERE condition does not contain DATEDIFF[^] function, because you can limit your data by compare VisitDate to the final date (stored in a @finaldate variable).

Above query could be - probably - improved more, but i have no access to your data and don't see your data structure...

As to the linq query...
There's no corresponding [IN] clause in Linq, but... we can use Where[^] together with Any[^] method. See:

int[] vid2get = new int[]{4,7,2,8,9};
var qry1 = PatVisit_context
	.Where(x=> vid2get.Any(y=>y==x.VisitId));

So, the final version of linq query may look like:

int[] vid2get = new int[]{4,7,2,8,9};
DateTime finaldate = DateTime.Now.AddDays(-10);
var qry1 = (from pv in PatVisit_context
	.Where(x=> vid2get.Any(y=>y==x.VisitId) && x.VisitDate >= finaldate)
	.GroupBy(x=> x.PatId)
	.Select(grp=> grp.OrderByDescending(y=>y.VisitDate).First())
	join d in Device_context on pv.PatId equals d.PatId && d.SyncDate != null
	select pv)
	.ToList();

Good luck!