It can allocate more :O

AllocationProof.cs

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

namespace AllocationBenchmark
{
    [DryCoreJob, DryClrJob]
    [MemoryDiagnoser]
    public class FormatAllocation
    {
       [Benchmark]
        public string FormatThreeParameters()
        {
            return $"a{1}a{2}a{3}a";
        }
        [Benchmark]
        public string FormatStringThreeParameters()
        {
            return $"b{1.ToString()}b{2.ToString()}b{3.ToString()}b";
        }
        [Benchmark]
        public string FormatOneParameter()
        {
            return $"a{1}a{2}a{3}a";
        }
        [Benchmark]
        public string FormatStringOneParameter()
        {
            return $"b{1.ToString()}b{2.ToString()}b{3.ToString()}b";
        }
        [Benchmark]
        public string FormatFourParameters()
        {
            return $"a{1}a{2}a{3}a{4}a";
        }
        [Benchmark]
        public string FormatStringFourParameters()
        {
            return $"b{1.ToString()}b{2.ToString()}b{3.ToString()}b{4.ToString()}b";
        }
        [Benchmark]
        public string FormatThreeParametersL()
        {
            return $"a{int.MaxValue}a{int.MaxValue}a{int.MaxValue}a";
        }
        [Benchmark]
        public string FormatStringThreeParametersL()
        {
            return $"b{int.MaxValue.ToString()}b{int.MaxValue.ToString()}b{int.MaxValue.ToString()}b";
        }
        [Benchmark]
        public string FormatOneParameterL()
        {
            return $"a{int.MaxValue}a{int.MaxValue}a{int.MaxValue}a";
        }
        [Benchmark]
        public string FormatStringOneParameterL()
        {
            return $"b{int.MaxValue.ToString()}b{int.MaxValue.ToString()}b{int.MaxValue.ToString()}b";
        }
        [Benchmark]
        public string FormatFourParametersL()
        {
            return $"a{int.MaxValue}a{int.MaxValue-1}a{int.MaxValue-2}a{int.MaxValue-3}a";
        }
        [Benchmark]
        public string FormatStringFourParametersL()
        {
            return $"b{(int.MaxValue - 1).ToString()}b{(int.MaxValue - 2).ToString()}b{(int.MaxValue - 3).ToString()}b{(int.MaxValue - 3).ToString()}b";
        }
        static void Main(string[] args)
        {
             BenchmarkRunner.Run<FormatAllocation>();
        }
    }
}

BenchmarkDotNet=v0.11.5, OS=Windows 10.0.18362
Intel Core i7-4790 CPU 3.60GHz (Haswell), 1 CPU, 8 logical and 4 physical cores
.NET Core SDK=3.0.100
  [Host]  : .NET Core 3.0.0 (CoreCLR 4.700.19.46205, CoreFX 4.700.19.46214), 64bit RyuJIT  [AttachedDebugger]
  DryClr  : .NET Framework 4.7.2 (CLR 4.0.30319.42000), 64bit RyuJIT-v4.8.4018.0
  DryCore : .NET Core 3.0.0 (CoreCLR 4.700.19.46205, CoreFX 4.700.19.46214), 64bit RyuJIT

IterationCount=1  LaunchCount=1  RunStrategy=ColdStart  
UnrollFactor=1  WarmupCount=1  

Method	Job	Runtime	Mean	Error	Gen 0	Gen 1	Gen 2	Allocated
FormatThreeParameters	DryClr	Clr	621.9 us	NA	-	-	-	8192 B
FormatStringThreeParameters	DryClr	Clr	595.6 us	NA	-	-	-	8192 B
FormatOneParameter	DryClr	Clr	780.5 us	NA	-	-	-	8192 B
FormatStringOneParameter	DryClr	Clr	538.4 us	NA	-	-	-	8192 B
FormatFourParameters	DryClr	Clr	596.8 us	NA	-	-	-	8192 B
FormatStringFourParameters	DryClr	Clr	557.7 us	NA	-	-	-	8192 B
FormatThreeParametersL	DryClr	Clr	622.9 us	NA	-	-	-	8192 B
FormatStringThreeParametersL	DryClr	Clr	529.0 us	NA	-	-	-	8192 B
FormatOneParameterL	DryClr	Clr	536.2 us	NA	-	-	-	8192 B
FormatStringOneParameterL	DryClr	Clr	523.2 us	NA	-	-	-	8192 B
FormatFourParametersL	DryClr	Clr	502.0 us	NA	-	-	-	8192 B
FormatStringFourParametersL	DryClr	Clr	556.1 us	NA	-	-	-	8192 B
FormatThreeParameters	DryCore	Core	439.6 us	NA	-	-	-	112 B
FormatStringThreeParameters	DryCore	Core	660.7 us	NA	-	-	-	120 B
FormatOneParameter	DryCore	Core	411.1 us	NA	-	-	-	112 B
FormatStringOneParameter	DryCore	Core	448.5 us	NA	-	-	-	120 B
FormatFourParameters	DryCore	Core	646.2 us	NA	-	-	-	192 B
FormatStringFourParameters	DryCore	Core	459.1 us	NA	-	-	-	136 B
FormatThreeParametersL	DryCore	Core	520.4 us	NA	-	-	-	168 B
FormatStringThreeParametersL	DryCore	Core	463.5 us	NA	-	-	-	320 B
FormatOneParameterL	DryCore	Core	452.5 us	NA	-	-	-	168 B
FormatStringOneParameterL	DryCore	Core	433.1 us	NA	-	-	-	320 B
FormatFourParametersL	DryCore	Core	433.8 us	NA	-	-	-	264 B
FormatStringFourParametersL	DryCore	Core	415.9 us	NA	-	-	-	400 B